Editing Color

{{Short description|Visual perception of the light spectrum}}
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{{Use American English|date=July 2020}}
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[[File:Colouring pencils.jpg|thumb|300x300px|[[Colored pencil]]s]]

'''Color''' (or '''colour''' in [[English in the Commonwealth of Nations|Commonwealth English]]; [[American and British English spelling differences#-our, -or|see spelling differences]]) is the [[visual perception]] based on the [[electromagnetic spectrum]].<!-- Don't add frequency, wavelength or photon energy as it is redundant. --> Though color is not an inherent property of [[matter]], color perception is related to an object's [[light absorption]], [[emission spectra|emission,]] [[Reflection (physics)|reflection]] and [[Transmittance|transmission]]. For most humans, colors are perceived in the visible [[light]] spectrum with three types of [[cone cell]]s ([[trichromacy]]). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelengths, such as bees that can distinguish [[ultraviolet]], and thus have a different color sensitivity range. Animal perception of color originates from different light [[wavelength]] or [[spectral sensitivity]] in cone cell types, which is then processed by the [[brain]].

Colors have perceived properties such as [[hue]], [[colorfulness]] (saturation), and [[luminance]]. Colors can also be [[additive color|additively mixed]] (commonly used for actual light) or [[subtractive color|subtractively mixed]] (commonly used for materials). If the colors are mixed in the right proportions, because of [[metamerism (color)|metamerism]], they may look the same as a single-wavelength light. For convenience, colors can be organized in a [[color space]], which when being abstracted as a mathematical [[color model]] can assign each region of color with a corresponding set of numbers. As such, color spaces are an essential tool for [[color reproduction]] in [[color printing|print]], [[color photography|photography]], computer monitors, and [[color television|television]]. Some of the most well-known color models and color spaces are [[RGB]], [[CMYK]], [[HSL and HSV|HSL/HSV]], [[CIE Lab]], and [[YCbCr]]/[[YUV]].

Because the perception of color is an important aspect of human life, different colors have been associated with [[emotion]]s, activity, and [[nationality]]. Names of [[color term|color regions]] in different cultures can have different, sometimes overlapping areas. In [[visual arts]], [[color theory]] is used to govern the use of colors in an [[aesthetics|aesthetically pleasing]] and [[harmony (color)|harmonious]] way. The theory of color includes the [[complementary colors|color complements]]; [[color balance]]; and classification of [[primary color]]s, [[secondary color]]s, and [[tertiary color]]s. The study of colors in general is called [[color science]].

== Physical properties ==

[[File:Visible spectrum 390-710 nm linear perceptual.svg|alt=gray fading to rainbow colors (red to violet), then fade back to gray|center|thumb|600x600px|The visible spectrum perceived from 390 to 710&nbsp;nm [[wavelength]]]]
[[Electromagnetic radiation]] is characterized by its [[wavelength]] (or [[frequency]]) and its [[luminous intensity|intensity]]. When the wavelength is within the [[visible spectrum]] (the range of wavelengths humans can perceive, approximately from 390&nbsp;[[nanometre|nm]] to 700&nbsp;nm), it is known as "visible [[light]]".<ref name="Bettini">{{cite book |last1=Bettini |first1=Alessandro |url=https://books.google.com/books?id=Ip9xDQAAQBAJ&q=%22electromagnetic+waves%22+charges+accelerating&pg=PA95 |title=A Course in Classical Physics, Vol. 4 – Waves and Light |date=2016 |publisher=Springer |isbn=978-3-319-48329-0 |pages=95, 103}}</ref>

Most light sources emit light at many different wavelengths; a source's ''spectrum'' is a distribution giving its intensity at each wavelength. Although the spectrum of light arriving at the eye from a given direction determines the color [[wikt:sensation|sensation]] in that direction, there are many more possible spectral combinations than color sensations. In fact, one may formally define a color as a class of spectra that give rise to the same color sensation, although such classes would vary widely among different species, and to a lesser extent among individuals within the same species. In each such class, the members are called ''[[metamerism (color)|metamer]]s'' of the color in question. This effect can be visualized by comparing the light sources' [[spectral power distribution]]s and the resulting colors.

=== Spectral colors<span class="anchor" id="Spectral colours"></span> ===
{{main|Spectral color}}
The familiar colors of the [[rainbow]] in the [[visible spectrum|spectrum]]—named using the [[Latin]] word for ''appearance'' or ''apparition'' by [[Isaac Newton]] in 1671—include all those colors that can be produced by visible [[light]] of a single wavelength only, the [[spectral color|''pure spectral'' or ''monochromatic'' color]]s. The spectrum above shows approximate wavelengths (in [[nanometre|nm]]) for spectral colors in the visible range. Spectral colors have 100% [[colorfulness#Excitation purity|purity]], and are fully [[colorfulness|saturate]]d. A complex mixture of spectral colors can be used to describe any color, which is the definition of a light [[power spectrum]].

The spectral colors form a continuous spectrum, and how it is divided into [[color term|distinct colors linguistically]] is a matter of culture and historical contingency.<ref>[[Brent Berlin|Berlin, B.]] and [[Paul Kay|Kay, P.]], ''[[Basic Color Terms: Their Universality and Evolution]]'', Berkeley: [[University of California Press]], 1969.</ref> Despite the ubiquitous [[ROYGBIV]] mnemonic used to remember the spectral colors in English, the inclusion or exclusion of colors is contentious, with disagreement often focused on [[indigo#Classification as a spectral color|indigo]] and cyan.<ref>{{cite book|last=Waldman|first=Gary|title=Introduction to light: the physics of light, vision, and color|year=2002|publisher=Dover Publications|location=Mineola|isbn=978-0486421186|page=193|url=https://books.google.com/books?id=PbsoAXWbnr4C&pg=PA193}}</ref> Even if the subset of color terms is agreed, their wavelength ranges and borders between them may not be.

The ''intensity'' of a spectral color, relative to the context in which it is viewed, may alter its perception considerably. For example, a low-intensity orange-yellow is [[brown]], and a low-intensity yellow-green is [[olive green]]. Additionally, hue shifts towards yellow or blue happen if the intensity of a spectral light is increased; this is called [[Bezold–Brücke shift]]. In [[color models]] capable of representing spectral colors,<ref>{{cite web |title=Perceiving Color |url=http://courses.washington.edu/psy333/lecture_pdfs/chapter7_Color.pdf#page=72 |website=courses.washington.edu}}</ref> such as [[CIELUV]], a spectral color has the maximal saturation. In [[Helmholtz–Kohlrausch effect#Helmholtz color coordinates|Helmholtz coordinates]], this is described as 100% [[Colorfulness#Excitation purity|purity]].

=== Color of objects<span class="anchor" id="Colour of objects"></span> ===

The physical color of an object depends on how it [[absorbance|absorb]]s and [[scattering|scatter]]s light. Most objects scatter light to some degree and do not reflect or transmit light [[specular]]ly like [[glass]]es or [[mirror]]s. A [[transparency (optics)|transparent]] object allows almost all light to [[transmittance|transmit]] or pass through, thus transparent objects are perceived as colorless. Conversely, an [[opacity (optics)|opaque]] object does not allow light to transmit through and instead absorbs or [[reflection (physics)|reflects]] the light it receives. Like transparent objects, [[translucent]] objects allow light to transmit through, but translucent objects are seen colored because they scatter or absorb certain wavelengths of light via internal scattering. The absorbed light is often dissipated as [[heat]].<ref name=":0">{{Cite book |last=Berns |first=Roy S. |url= |title=Billmeyer and Saltzman's Principles of Color Technology |publisher=[[Wiley (publisher)|Wiley]] |others=Fred W. Billmeyer, Max Saltzman |year=2019 |isbn=978-1119366683 |edition=4th |location=Hoboken, NJ |oclc=1080250734|pages=5–9, 12}}</ref>

== Color vision<span class="anchor" id="Colour vision"></span> ==
{{main|Color vision}}

=== Development of theories of color vision<span class="anchor" id="Development of theories of colour vision"></span> ===
{{main|Color theory}}
[[File:Optical grey squares orange brown.svg|right|thumb|250px|The upper disk and the lower disk have exactly the same objective color, and are in identical gray surroundings; based on context differences, humans perceive the squares as having different reflectances, and may interpret the colors as different color categories; see [[checker shadow illusion]]]]
Although [[Aristotle]] and other ancient scientists had already written on the nature of light and [[color vision]], it was not until [[Isaac Newton|Newton]] that light was identified as the source of the color sensation. In 1810, [[Goethe]] published his comprehensive ''[[Theory of Colors]]'' in which he provided a rational description of color experience, which 'tells us how it originates, not what it is'. (Schopenhauer)

In 1801 [[Thomas Young (scientist)|Thomas Young]] proposed his [[trichromacy|trichromatic theory]], based on the observation that any color could be matched with a combination of three lights. This theory was later refined by [[James Clerk Maxwell]] and [[Hermann von Helmholtz]]. As Helmholtz puts it, "the principles of Newton's law of mixture were experimentally confirmed by Maxwell in 1856. Young's theory of color sensations, like so much else that this marvelous investigator achieved in advance of his time, remained unnoticed until Maxwell directed attention to it."<ref>Hermann von Helmholtz, ''Physiological Optics: The Sensations of Vision'', 1866, as translated in ''Sources of Color Science'', David L. MacAdam, ed., Cambridge: [[MIT Press]], 1970.</ref>

At the same time as Helmholtz, [[Ewald Hering]] developed the [[opponent process]] theory of color, noting that [[color blindness]] and afterimages typically come in opponent pairs (red-green, blue-orange, yellow-violet, and black-white). Ultimately these two theories were synthesized in 1957 by Hurvich and Jameson, who showed that retinal processing corresponds to the trichromatic theory, while processing at the level of the [[lateral geniculate nucleus]] corresponds to the opponent theory.<ref>Palmer, S.E. (1999). ''Vision Science: Photons to Phenomenology'', Cambridge, MA: MIT Press. {{ISBN|0262161834}}.</ref>

In 1931, an international group of experts known as the ''Commission internationale de l'éclairage'' ([[International Commission on Illumination|CIE]]) developed a mathematical color model, which mapped out the space of observable colors and assigned a set of three numbers to each.

=== Color in the eye<span class="anchor" id="Colour in the eye"></span> ===
{{main|Color vision#Cone cells in the human eye}}
[[File:Cones SMJ2 E.svg|thumb|upright=1.2|Normalized typical human [[cone cell]] responses (''S'', ''M'', and ''L types'') to monochromatic spectral stimuli]]

The ability of the [[human eye]] to distinguish colors is based upon the varying sensitivity of different cells in the [[retina]] to light of different [[wavelength]]s. Humans are [[trichromatic]]—the retina contains three types of color receptor cells, or [[cone cell|cone]]s. One type, relatively distinct from the other two, is most responsive to light that is perceived as blue or blue-violet, with wavelengths around 450&nbsp;[[nanometre|nm]]; cones of this type are sometimes called ''short-wavelength cones'' or ''S cones'' (or misleadingly, ''blue cones''). The other two types are closely related genetically and chemically: ''middle-wavelength cones'', ''M cones'', or ''green cones'' are most sensitive to light perceived as green, with wavelengths around 540&nbsp;nm, while the ''long-wavelength cones'', ''L cones'', or ''red cones'', are most sensitive to light that is perceived as greenish yellow, with wavelengths around 570&nbsp;nm.

Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. Each cone type adheres to the [[principle of univariance]], which is that each cone's output is determined by the amount of light that falls on it over all wavelengths. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These amounts of stimulation are sometimes called ''tristimulus values''.<ref>{{Cite web |last=Magazine |first=Nicola Jones, Knowable |title=Color Is in the Eye, and Brain, of the Beholder |url=https://www.scientificamerican.com/article/color-is-in-the-eye-and-brain-of-the-beholder/ |access-date=2022-11-08 |website=Scientific American |language=en}}</ref>

The response curve as a function of wavelength varies for each type of cone. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. For example, it is not possible to stimulate ''only'' the mid-wavelength (so-called "green") cones; the other cones will inevitably be stimulated to some degree at the same time. The set of all possible tristimulus values determines the human ''color space''. It has been estimated that humans can distinguish roughly 10 million different colors.<ref name="business">{{cite book|last1=Judd|first1=Deane B.|title=Color in Business, Science and Industry|last2=Wyszecki|first2=Günter|publisher=[[Wiley-Interscience]]|year=1975|isbn=978-0471452126|edition=3rd|series=Wiley Series in Pure and Applied Optics|location=New York|page=388}}</ref>

The other type of light-sensitive cell in the eye, the [[rod cell|rod]], has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all.<ref>"Under well-lit viewing conditions (photopic vision), cones&nbsp; ...are highly active and rods are inactive."{{cite conference|last=Hirakawa|first=K.|author2=Parks, T.W.|title=IEEE International Conference on Image Processing 2005|chapter=Chromatic Adaptation and White-Balance Problem|conference=IEEE ICIP|year=2005|pages=iii-984|doi=10.1109/ICIP.2005.1530559|isbn=0780391349|chapter-url=http://www.accidentalmark.com/research/papers/Hirakawa05WBICIP.pdf|url-status=dead|archive-url=https://web.archive.org/web/20061128184104/http://www.accidentalmark.com/research/papers/Hirakawa05WBICIP.pdf|archive-date=November 28, 2006}}</ref> On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a [[black-and-white|colorless]] response (furthermore, the rods are barely sensitive to light in the "red" range). In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone. These effects, combined, are summarized also in the [[Kruithof curve]], which describes the change of color perception and pleasingness of light as a function of temperature and intensity.

=== Color in the brain<span class="anchor" id="Colour in the brain"></span> ===
{{main|Color vision#Color in the primate brain}}
<!--opponent process is not calculated in the brain, but still in neurons in the retina; this section needs to focus more on the visual cortex-->
While the mechanisms of [[color vision]] at the level of the [[retina]] are well-described in terms of tristimulus values, color processing after that point is organized differently. A dominant theory of color vision proposes that color information is transmitted out of the eye by three [[opponent process]]es, or opponent channels, each constructed from the raw output of the cones: a red–green channel, a blue–yellow channel, and a black–white "luminance" channel. This theory has been supported by neurobiology, and accounts for the structure of our subjective color experience. Specifically, it explains why humans cannot perceive a "reddish green" or "yellowish blue", and it predicts the [[color wheel]]: it is the collection of colors for which at least one of the two color channels measures a value at one of its extremes.

The exact nature of color perception beyond the processing already described, and indeed the status of color as a feature of the perceived world or rather as a feature of our ''perception'' of the world—a type of [[qualia]]—is a matter of complex and continuing philosophical dispute.{{citation needed|date=November 2022}}
[[File:Ventral-dorsal streams.svg|thumb|upright=1.25|The visual [[two-streams hypothesis#Dorsal stream|dorsal stream]] (green) and [[ventral stream]] (purple) are shown; the ventral stream is responsible for color perception]]
From the V1 blobs, color information is sent to cells in the second visual area, V2. The cells in V2 that are most strongly color tuned are clustered in the "thin stripes" that, like the blobs in V1, stain for the enzyme cytochrome oxidase (separating the thin stripes are interstripes and thick stripes, which seem to be concerned with other visual information like motion and high-resolution form). Neurons in V2 then synapse onto cells in the extended V4. This area includes not only V4, but two other areas in the posterior inferior temporal cortex, anterior to area V3, the dorsal posterior inferior temporal cortex, and posterior TEO.<ref name="Conway_2007">{{cite journal |vauthors=Conway BR, Moeller S, Tsao DY |date=November 2007 |title=Specialized color modules in macaque extrastriate cortex |url=https://authors.library.caltech.edu/100800/ |journal=Neuron |volume=56 |issue=3 |pages=560–73 |doi=10.1016/j.neuron.2007.10.008 |pmc=8162777 |pmid=17988638 |s2cid=11724926 |access-date=2023-12-08 |archive-date=2022-10-10 |archive-url=https://web.archive.org/web/20221010104403/https://authors.library.caltech.edu/100800/ |url-status=dead  |issn = 0896-6273}}</ref><ref name="Conway_2009">{{cite journal |vauthors=Conway BR, Tsao DY |date=October 2009 |title=Color-tuned neurons are spatially clustered according to color preference within alert macaque posterior inferior temporal cortex |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=106 |issue=42 |pages=18034–9 |bibcode=2009PNAS..10618034C |doi=10.1073/pnas.0810943106 |pmc=2764907 |pmid=19805195 |doi-access=free}}</ref> Area V4 was initially suggested by [[Semir Zeki]] to be exclusively dedicated to color,<ref>{{cite journal |vauthors=Zeki SM |date=April 1973 |title=Colour coding in rhesus monkey prestriate cortex |journal=Brain Research |volume=53 |issue=2 |pages=422–7 |doi=10.1016/0006-8993(73)90227-8 |pmid=4196224}}</ref> and he later showed that V4 can be subdivided into subregions with very high concentrations of color cells separated from each other by zones with lower concentration of such cells though even the latter cells respond better to some wavelengths than to others,<ref name="Zeki_1983">{{cite journal |vauthors=Zeki S |date=March 1983 |title=The distribution of wavelength and orientation selective cells in different areas of monkey visual cortex |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=217 |issue=1209 |pages=449–70 |bibcode=1983RSPSB.217..449Z |doi=10.1098/rspb.1983.0020 |pmid=6134287 |s2cid=39700958}}</ref> a finding confirmed by subsequent studies.<ref name="Conway_2007" /><ref>{{cite journal |vauthors=Bushnell BN, Harding PJ, Kosai Y, Bair W, Pasupathy A |date=August 2011 |title=Equiluminance cells in visual cortical area v4 |journal=The Journal of Neuroscience |volume=31 |issue=35 |pages=12398–412 |doi=10.1523/JNEUROSCI.1890-11.2011 |pmc=3171995 |pmid=21880901}}</ref><ref>{{cite journal |vauthors=Tanigawa H, Lu HD, Roe AW |date=December 2010 |title=Functional organization for color and orientation in macaque V4 |journal=Nature Neuroscience |volume=13 |issue=12 |pages=1542–8 |doi=10.1038/nn.2676 |pmc=3005205 |pmid=21076422}}</ref> The presence in V4 of orientation-selective cells led to the view that V4 is involved in processing both color and form associated with color<ref name="Zeki_2005">{{cite journal |vauthors=Zeki S |date=June 2005 |title=The Ferrier Lecture 1995 behind the seen: the functional specialization of the brain in space and time |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=360 |issue=1458 |pages=1145–83 |doi=10.1098/rstb.2005.1666 |pmc=1609195 |pmid=16147515}}</ref> but it is worth noting that the orientation selective cells within V4 are more broadly tuned than their counterparts in V1, V2, and V3.<ref name="Zeki_1983" /> Color processing in the extended V4 occurs in millimeter-sized color modules called [[Glob (visual system)|globs]].<ref name="Conway_2007" /><ref name="Conway_2009" /> This is the part of the brain in which color is first processed into the full range of [[hue]]s found in [[color space]].<ref>{{Cite journal |last=Zeki |first=S. |date=1980 |title=The representation of colours in the cerebral cortex |url=https://www.nature.com/articles/284412a0 |journal=Nature |language=en |volume=284 |issue=5755 |pages=412–418 |bibcode=1980Natur.284..412Z |doi=10.1038/284412a0 |issn=1476-4687 |pmid=6767195 |s2cid=4310049}}</ref><ref name="Conway_2007" /><ref name="Conway_2009" />

=== Nonstandard color perception<span class="anchor" id="Nonstandard colour perception"></span> ===

==== Color vision deficiency<span class="anchor" id="Colour vision deficiency"></span> ====
{{main|Color blindness}}
A color vision deficiency causes an individual to perceive a smaller [[gamut]] of colors than the standard observer with normal color vision. The effect can be mild, having lower "color resolution" (i.e. [[anomalous trichromacy]]), moderate, lacking an entire dimension or channel of color (e.g. [[dichromacy]]), or complete, lacking all color perception (i.e. [[monochromacy]]). Most forms of color blindness derive from one or more of the three classes of cone cells either being missing, having a shifted [[spectral sensitivity]] or having lower responsiveness to incoming light. In addition, [[cerebral achromatopsia]] is caused by neural anomalies in those parts of the brain where visual processing takes place.

Some colors that appear distinct to an individual with normal color vision will appear [[metamerism (color)|metameric]] to the color blind. The most common form of color blindness is [[congenital red–green color blindness]], affecting ~8% of males. Individuals with the strongest form of this condition ([[dichromacy]]) will experience blue and purple, green and yellow, teal, and gray as colors of confusion, i.e. metamers.<ref>{{cite web |last1=Flück |first1=Daniel |title=Colorblind colors of confusion |url=https://www.color-blindness.com/2009/01/19/colorblind-colors-of-confusion/ |website=Colblindor |date=19 January 2009 |access-date=14 November 2022}}</ref>

==== Tetrachromacy ====
{{main|Tetrachromacy}}
Outside of humans, which are mostly ''trichromatic'' (having three types of cones), most mammals are dichromatic, possessing only two cones. However, outside of mammals, most vertebrates are ''[[tetrachromatic]]'', having four types of cones. This includes most [[bird]]s,<ref>{{cite journal |last1=Bennett |first1=Andrew T. D. |last2=Cuthill |first2=Innes C. |last3=Partridge |first3=Julian C. |last4=Maier |first4=Erhard J. |year=1996 |title=Ultraviolet vision and mate choice in zebra finches |journal=Nature |volume=380 |issue=6573 |pages=433–435 |bibcode=1996Natur.380..433B |doi=10.1038/380433a0 |s2cid=4347875}}</ref><ref>{{cite journal |last1=Bennett |first1=Andrew T. D. |last2=Théry |first2=Marc |year=2007 |title=Avian Color Vision and Coloration: Multidisciplinary Evolutionary Biology |url=https://hal.archives-ouvertes.fr/hal-02889396/file/Bennett%20%26%20Thery%20Am%20Nat%202007.pdf |journal=The American Naturalist |volume=169 |issue=S1 |pages=S1–S6 |doi=10.1086/510163 |issn=0003-0147 |jstor=510163 |bibcode=2007ANat..169S...1B |s2cid=2484928}}</ref><ref>{{cite book |last1=Cuthill |first1=Innes C. |title=Ultraviolet Vision in Birds |last2=Partridge |first2=Julian C. |last3=Bennett |first3=Andrew T. D. |last4=Church |first4=Stuart C. |last5=Hart |first5=Nathan S. |last6=Hunt |first6=Sarah |date=2000 |publisher=Academic Press |isbn=978-0-12-004529-7 |editor1-last=J. B. Slater |editor1-first=Peter |series=Advances in the Study of Behavior |volume=29 |page=159 |doi=10.1016/S0065-3454(08)60105-9 |editor2-last=Rosenblatt |editor2-first=Jay S. |editor3-last=Snowdon |editor3-first=Charles T. |editor4-last=Roper |editor4-first=Timothy J.}}</ref> [[reptile]]s, [[amphibian]]s, and [[teleost|bony fish]].<ref name="Bowm1">{{cite journal |last1=Bowmaker |first1=James K. |date=September 2008 |title=Evolution of vertebrate visual pigments |journal=Vision Research |volume=48 |issue=20 |pages=2022–2041 |doi=10.1016/j.visres.2008.03.025 |pmid=18590925 |s2cid=52808112 |doi-access=free}}</ref><ref>{{cite journal |last=Vorobyev |first=M. |date=November 1998 |title=Tetrachromacy, oil droplets and bird plumage colours |journal=Journal of Comparative Physiology A |volume=183 |issue=5 |pages=621–33 |doi=10.1007/s003590050286 |pmid=9839454 |s2cid=372159}}</ref> An extra dimension of color vision means these vertebrates can see two distinct colors that a normal human would view as [[metamerism (color)|metamer]]s. Some invertebrates, such as the [[mantis shrimp]], have an even higher number of cones (12) that could lead to a richer color [[gamut]] than even imaginable by humans.

The existence of human tetrachromats is a contentious notion. As many as [[tetrachromacy#Tetrachromacy in carriers of CVD|half of all human females have 4 distinct cone classes]], which could enable tetrachromacy.<ref name="Jameson">{{cite journal|last1=Jameson|first1=K.A.|last2=Highnote|first2=S.M.|last3=Wasserman|first3=L.M.|year=2001|title=Richer color experience in observers with multiple photopigment opsin genes.|doi=10.3758/BF03196159|journal=Psychonomic Bulletin and Review|volume=8|issue=2|pages=244–261 [256]|url=https://link.springer.com/content/pdf/10.3758/BF03196159.pdf |archive-url=https://web.archive.org/web/20131004220637/http://link.springer.com/content/pdf/10.3758/BF03196159.pdf |archive-date=2013-10-04 |url-status=live|pmid=11495112|s2cid=2389566|doi-access=free}}</ref> However, a distinction must be made between ''retinal'' (or ''weak'') ''tetrachromats'', which express four cone classes in the retina, and ''functional'' (or ''strong'') ''tetrachromats'', which are able to make the enhanced color discriminations expected of tetrachromats. In fact, there is only one peer-reviewed report of a functional tetrachromat.<ref>{{cite journal|last1=Jordan|first1=G.|last2=Deeb|first2=S.S.|last3=Bosten|first3=J.M.|last4=Mollon|first4=J.D.|title=The dimensionality of color vision in carriers of anomalous trichromacy|journal=Journal of Vision|date=20 July 2010|volume=10|issue=8|page=12|doi=10.1167/10.8.12|pmid=20884587|doi-access=free}}</ref> It is estimated that while the average person is able to see one&nbsp;million colors, someone with functional tetrachromacy could see a hundred&nbsp;million colors.<ref>{{cite web|last=Kershner|first=Kate|title=Lucky Tetrachromats See World With Up to 100 Million Colors|date=26 July 2016|url=https://science.howstuffworks.com/lucky-tetrachromats-see-world-100-million-colors.htm|access-date=9 February 2022}}</ref>

==== Synesthesia ====
{{main|Synesthesia}}

In certain forms of [[synesthesia]], perceiving letters and numbers ([[grapheme–color synesthesia]]) or hearing sounds ([[chromesthesia]]) will evoke a perception of color. Behavioral and [[functional neuroimaging]] experiments have demonstrated that these color experiences lead to changes in behavioral tasks and lead to increased activation of brain regions involved in color perception, thus demonstrating their reality, and similarity to real color percepts, albeit evoked through a non-standard route. Synesthesia can occur genetically, with 4% of the population having variants associated with the condition. Synesthesia has also been known to occur with brain damage, drugs, and sensory deprivation.<ref>{{cite journal|last1=Brang|first1=David|title=Survival of the Synesthesia Gene: Why Do People Hear Colors and Taste Words?|journal=PLOS Biology|date=22 November 2011|volume=9|issue=11|pages=e1001205|doi=10.1371/journal.pbio.1001205|pmid=22131906|pmc=3222625|doi-access=free}}</ref>

The philosopher Pythagoras experienced synesthesia and provided one of the first written accounts of the condition in approximately 550&nbsp;BCE. He created mathematical equations for musical notes that could form part of a scale, such as an octave.<ref>{{cite web|title=A Brief History of Synesthesia in the Arts|url=http://www.daysyn.com/history.html|access-date=9 February 2022}}</ref>

=== Afterimages ===
{{main|Afterimage}}

After exposure to strong light in their sensitivity range, [[photoreceptor cell|photoreceptor]]s of a given type become desensitized.<ref>{{cite journal |last1=Gersztenkorn |first1=D |last2=Lee |first2=AG |date=Jul 2, 2014 |title=Palinopsia revamped: A systematic review of the literature. |journal=Survey of Ophthalmology |volume=60 |issue=1 |pages=1–35 |doi=10.1016/j.survophthal.2014.06.003 |pmid=25113609}}</ref><ref>{{cite journal |last1=Bender |first1=MB |last2=Feldman |first2=M |last3=Sobin |first3=AJ |date=Jun 1968 |title=Palinopsia. |journal=Brain: A Journal of Neurology |volume=91 |issue=2 |pages=321–38 |doi=10.1093/brain/91.2.321 |pmid=5721933}}</ref> For a few seconds after the light ceases, they will continue to signal less strongly than they otherwise would. Colors observed during that period will appear to lack the color component detected by the desensitized photoreceptors. This effect is responsible for the phenomenon of [[afterimage]]s, in which the eye may continue to see a bright figure after looking away from it, but in a [[complementary color]]. Afterimage effects have also been used by artists, including [[Vincent van Gogh]].

=== Color constancy<span class="anchor" id="Colour constancy"></span> ===
{{main|Color constancy}}
When an artist uses a limited [[color palette]], the human [[visual system]] tends to compensate by seeing any gray or neutral color as the color which is missing from the color wheel. For example, in a limited palette consisting of red, yellow, black, and white, a mixture of yellow and black will appear as a variety of green, a mixture of red and black will appear as a variety of purple, and pure gray will appear bluish.<ref>{{cite web|last=Depauw|first=Robert C.|title=United States Patent|url=http://www.google.com/patents?hl=en&lr=&vid=USPAT3815265&id=tSEzAAAAEBAJ&oi=fnd&dq=mixing+paint+colors&printsec=abstract#v=onepage&q=mixing%20paint%20colors&f=false|access-date=20 March 2011|archive-date=6 January 2012|archive-url=https://web.archive.org/web/20120106111021/http://www.google.com/patents?hl=en&lr=&vid=USPAT3815265&id=tSEzAAAAEBAJ&oi=fnd&dq=mixing+paint+colors&printsec=abstract#v=onepage&q=mixing%20paint%20colors&f=false|url-status=dead}}</ref><!-- not due to black pigment being dark blue therefore reflecting more blue light? This is a real physical phenomenon and not a perceptual one. Is this paragraph not irrelevant to color constancy? -->

The trichromatic theory is strictly true when the visual system is in a fixed state of adaptation.<ref>{{Cite journal |last=Walters |first=H. V. |date=1942 |title=Some Experiments on the Trichromatic Theory of Vision |url=https://www.jstor.org/stable/82365 |journal=Proceedings of the Royal Society of London. Series B, Biological Sciences |volume=131 |issue=862 |pages=27–50 |doi=10.1098/rspb.1942.0016 |jstor=82365 |bibcode=1942RSPSB.131...27W |s2cid=120320368 |issn=0080-4649}}</ref> In reality, the visual system is constantly adapting to changes in the environment and compares the various colors in a scene to reduce the effects of the illumination. If a scene is illuminated with one light, and then with another, as long as the difference between the light sources stays within a reasonable range, the colors in the scene appear relatively constant to us. This was studied by [[Edwin H. Land]] in the 1970s and led to his retinex theory of [[color constancy]].<ref>{{Cite web |title=Edwin H. Land {{!}} Optica |url=https://www.optica.org/History/Biographies/bios/Edwin-H--Land |access-date=2023-12-08 |website=www.optica.org}}</ref><ref>{{Cite journal |last=Campbell |first=F. W. |date=1994 |title=Edwin Herbert Land. 7 May 1909-1 March 1991 |url=https://www.jstor.org/stable/770305 |journal=Biographical Memoirs of Fellows of the Royal Society |volume=40 |pages=197–219 |doi=10.1098/rsbm.1994.0035 |jstor=770305 |s2cid=72500555 |issn=0080-4606}}</ref>

Both phenomena are readily explained and mathematically modeled with modern theories of chromatic adaptation and color appearance (e.g. [[CIECAM02]], iCAM).<ref name="CAM">M.D. Fairchild, [http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470012161.html Color Appearance Models] {{webarchive|url=https://web.archive.org/web/20110505034940/http://www.wiley.com/WileyCDA/WileyTitle/productCd-0470012161.html|date=May 5, 2011}}, 2nd Ed., Wiley, Chichester (2005).</ref> There is no need to dismiss the trichromatic theory of vision, but rather it can be enhanced with an understanding of how the visual system adapts to changes in the viewing environment.

== Reproduction ==

{{main|Color reproduction}}

[[File:CIE chromaticity diagram 2012 version.png|thumb|upright=1.25|The [[CIE 1931 color space]] xy [[chromaticity]] diagram with the visual locus plotted using the CIE (2006) physiologically relevant LMS fundamental color matching functions transformed into the CIE 1931 xy [[color space]] and converted into [[Adobe RGB]]; the triangle shows the [[gamut]] of Adobe RGB, the [[Planckian locus]] is shown with color temperatures labeled in [[Kelvin]]s, the outer curved boundary is the spectral (or monochromatic) locus, with wavelengths shown in nanometers, the colors in this file are being specified using Adobe RGB, areas outside the triangle cannot be accurately rendered since they are outside the gamut of Adobe RGB, therefore they have been interpreted, the colors depicted depend on the gamut and color accuracy of your display]]

Color reproduction is the science of creating colors for the human eye that faithfully represent the desired color. It focuses on how to construct a spectrum of wavelengths that will best evoke a certain color in an observer. Most colors are not [[#Spectral colors|spectral color]]s, meaning they are mixtures of various wavelengths of light. However, these non-spectral colors are often described by their [[dominant wavelength]], which identifies the single wavelength of light that produces a sensation most similar to the non-spectral color. Dominant wavelength is roughly akin to [[hue]].

There are many color perceptions that by definition cannot be pure spectral colors due to [[colorfulness|desaturation]] or because they are [[purple]]s (mixtures of red and violet light, from opposite ends of the spectrum). Some examples of necessarily non-spectral colors are the achromatic colors ([[black]], [[gray]], and [[white]]) and colors such as [[pink]], [[tan (color)|tan]], and [[magenta]].

Two different light spectra that have the same effect on the three color receptors in the human eye will be perceived as the same color. They are [[metamerism (color)|metamer]]s of that color. This is exemplified by the white light emitted by fluorescent lamps, which typically has a spectrum of a few narrow bands, while daylight has a continuous spectrum. The human eye cannot tell the difference between such light spectra just by looking into the light source, although the [[color rendering index]] of each light source may affect the color of objects illuminated by these metameric light sources.

Similarly, most human color perceptions can be generated by a mixture of three colors called ''primaries''. This is used to reproduce color scenes in photography, printing, television, and other media. There are a number of methods or [[color space]]s for specifying a color in terms of three particular [[primary color]]s. Each method has its advantages and disadvantages depending on the particular application.

No mixture of colors, however, can produce a response truly identical to that of a spectral color, although one can get close, especially for the longer wavelengths, where the [[CIE 1931 color space]] chromaticity diagram has a nearly straight edge. For example, mixing green light (530&nbsp;nm) and blue light (460&nbsp;nm) produces cyan light that is slightly desaturated, because response of the red color receptor would be greater to the green and blue light in the mixture than it would be to a pure cyan light at 485&nbsp;nm that has the same intensity as the mixture of blue and green.

Because of this, and because the ''primaries'' in [[color printing]] systems generally are not pure themselves, the colors reproduced are never perfectly saturated spectral colors, and so spectral colors cannot be matched exactly. However, natural scenes rarely contain fully saturated colors, thus such scenes can usually be approximated well by these systems. The range of colors that can be reproduced with a given color reproduction system is called the [[gamut]]. The [[International Commission on Illumination|CIE]] chromaticity diagram can be used to describe the gamut.

Another problem with color reproduction systems is connected with the initial measurement of color, or [[colorimetry]]. The characteristics of the color sensors in measurement devices (e.g. cameras, scanners) are often very far from the characteristics of the receptors in the human eye.

A color reproduction system "tuned" to a human with normal color vision may give very inaccurate results for other observers, according to color vision deviations to the standard observer.

The different color response of different devices can be problematic if not properly managed. For color information stored and transferred in digital form, [[color management]] techniques, such as those based on [[ICC profile]]s, can help to avoid distortions of the reproduced colors. Color management does not circumvent the gamut limitations of particular output devices, but can assist in finding good mapping of input colors into the gamut that can be reproduced.

=== Additive coloring<span class="anchor" id="Additive colouring"></span> ===
[[File:AdditiveColor.svg|thumb|Additive color mixing: combining red and green yields yellow; combining all three primary colors together yields white]]
[[Additive color]] is light created by mixing together [[light]] of two or more different colors.<ref>{{cite web |last1=MacEvoy |first1=Bruce |title=handprint : colormaking attributes |url=https://www.handprint.com/HP/WCL/color5.html#theoryadd |access-date=26 February 2019 |website=www.handprint.com}}</ref><ref name="briggs">{{cite web |author=David Briggs |year=2007 |title=The Dimensions of Color |url=http://www.huevaluechroma.com/044.php |url-status=live |archive-url=https://web.archive.org/web/20150928031404/http://www.huevaluechroma.com/044.php |archive-date=2015-09-28 |access-date=2011-11-23}}</ref> [[Red]], [[green]], and [[blue]] are the additive [[primary color]]s normally used in additive color systems such as projectors, televisions, and computer terminals.

=== Subtractive coloring<span class="anchor" id="Subtractive colouring"></span> ===
[[File:SubtractiveColor.svg|thumb|Subtractive color mixing: combining yellow and magenta yields red; combining all three primary colors together yields black]]

[[Subtractive color]]ing uses dyes, inks, pigments, or filters to absorb some wavelengths of light and not others.<ref>{{Cite web |title=Molecular Expressions Microscopy Primer: Physics of Light and Color – Introduction to the Primary Colors |url=https://micro.magnet.fsu.edu/primer/lightandcolor/primarycolorsintro.html |access-date=2023-12-08 |website=micro.magnet.fsu.edu}}</ref> The color that a surface displays comes from the parts of the visible spectrum that are not absorbed and therefore remain visible. Without pigments or dye, fabric fibers, paint base and paper are usually made of particles that scatter white light (all colors) well in all directions. When a pigment or ink is added, wavelengths are absorbed or "subtracted" from white light, so light of another color reaches the eye.

If the light is not a pure white source (the case of nearly all forms of artificial lighting), the resulting spectrum will appear a slightly different color. [[Red]] paint, viewed under [[blue]] light, may appear [[black]]. Red paint is red because it scatters only the red components of the spectrum. If red paint is illuminated by blue light, it will be absorbed by the red paint, creating the appearance of a black object.

The subtractive model also predicts the color resulting from a mixture of paints, or similar medium such as fabric dye, whether applied in layers or mixed together prior to application. In the case of paint mixed before application, incident light interacts with many different pigment particles at various depths inside the paint layer before emerging.<ref>{{cite book |last1=Williamson |first1=Samuel J |title=Light and Color in Nature and Art |last2=Cummins |first2=Herman Z |date=1983 |publisher=John Wiley & Sons, Inc. |isbn=0-471-08374-7 |location=New York |pages=28–30 |quote="Thus subtractive color mixing laws that successfully describe how light is altered by nonspectral filters also describes how light is altered by pigments."}}</ref>

=== Structural color<span class="anchor" id="Structural colour"></span> ===
{{further|Structural coloration|Animal coloration}}
[[File:Peacock feathers closeup.jpg|thumb|The bright colors of Peacock feathers are caused by structural coloration]]
Structural colors are colors caused by interference effects rather than by pigments. Color effects are produced when a material is scored with fine parallel lines, formed of one or more parallel thin layers, or otherwise composed of microstructures on the scale of the color's [[wavelength]]. If the microstructures are spaced randomly, light of shorter wavelengths will be scattered preferentially to produce [[Tyndall effect]] colors: the blue of the sky (Rayleigh scattering, caused by structures much smaller than the wavelength of light, in this case, air molecules), the luster of [[opal]]s, and the blue of human irises. If the microstructures are aligned in arrays, for example, the array of pits in a CD, they behave as a [[diffraction grating]]: the grating reflects different wavelengths in different directions due to [[wave interference|interference]] phenomena, separating mixed "white" light into light of different wavelengths. If the structure is one or more thin layers then it will reflect some wavelengths and transmit others, depending on the layers' thickness.

Structural color is studied in the field of [[thin-film optics]]. The most ordered or the most changeable structural colors are [[iridescent]]. Structural color is responsible for the blues and greens of the feathers of many birds (the blue jay, for example), as well as certain butterfly wings and beetle shells. Variations in the pattern's spacing often give rise to an iridescent effect, as seen in [[peacock]] feathers, [[soap bubble]]s, films of oil, and [[mother of pearl]], because the reflected color depends upon the viewing angle. Numerous scientists have carried out research in butterfly wings and beetle shells, including Isaac Newton and Robert Hooke. Since 1942, [[electron microscope|electron micrography]] has been used, advancing the development of products that exploit structural color, such as "[[photonic]]" cosmetics.<ref>{{cite web|url=http://www.esrc.ac.uk/ESRCInfoCentre/about/CI/events/FSS/2006/science.aspx?ComponentId=14867&SourcePageId=14865|title=Economic and Social Research Council: Science in the Dock, Art in the Stocks|access-date=2007-10-07|url-status=dead|archive-url=https://web.archive.org/web/20071102025015/http://www.esrc.ac.uk/ESRCInfoCentre/about/CI/events/FSS/2006/science.aspx?ComponentId=14867&SourcePageId=14865|archive-date=November 2, 2007}}</ref>

== Optimal colors ==

{{main|Gamut#Surfaces (optimal colors)}}

Optimal colors are the most chromatic colors that surfaces can have. That is, optimal colors are the theoretical limit for the color of objects*.<ref name="new">{{cite journal |last1=Perales |first1=Esther |last2=Mora Estevan |first2=Teresa |last3=Viqueira Pérez |first3=Valentin |last4=de Fez |first4=Dolores |last5=Gilabert Pérez |first5=Eduardo José |last6=Martínez-Verdú |first6=Francisco M. |year=2005 |title=A new algorithm for calculating the MacAdam limits for any luminance factor, hue angle and illuminant |url=https://www.researchgate.net/publication/39435417 |journal=Repositorio Institucional de la Universidad de Alicante }}</ref> For now, we are unable to produce objects with such colors, at least not without recurring to more complex physical phenomena.

''*(with classical reflection. Phenomena like [[fluorescence]] or [[structural coloration]] may cause the color of objects to lie outside the optimal color solid)''

The plot of the gamut bounded by optimal colors in a color space is called the optimal [[color solid]] or [[Siegfried Rösch|Rösch]]–[[David MacAdam|MacAdam]] color solid.

The [[Reflectance|reflectance spectrum]] of a color is the amount of light of each wavelength that it reflects, in proportion to a given maximum, which is total reflection of light of that wavelength, and has the value of 1 (100%). If the reflectance spectrum of a color is 0 (0%) or 1 (100%) across the entire visible spectrum, and it has no more than two transitions between 0 and 1, or 1 and 0, then it is an optimal color. With the current state of technology, we are unable to produce any material or pigment with these properties.<ref name="sch">{{cite journal |last=Schrödinger |first=Erwin |year=1919 |title=Theorie der Pigmente größter Leuchtkraft |url=https://zenodo.org/record/1424357 |journal=Annalen der Physik |volume=367 |issue=15 |pages=603–622 |bibcode=1920AnP...367..603S |doi=10.1002/andp.19203671504}}</ref>

[[File:Rechteckspektrum_sRGB.svg|right|thumb|268x268px|Reflectance spectrum of a color-optimal reflective material. There is no known material with these properties, they are, for what we know, only theoretical.<ref name="thi">{{cite journal |last1=Koenderink |first1=Jan |last2=van Doorn |first2=Andrea J. |last3=Gegenfurtner |first3=Karl |year=2021 |title=RGB Colors and Ecological Optics |url=https://www.researchgate.net/publication/351231527 |journal=Frontiers in Computer Science |volume=3 |doi=10.3389/fcomp.2021.630370 |doi-access=free }}</ref>]]

Thus four types of "optimal color" spectra are possible: 
* The transition goes from zero at both ends of the spectrum to one in the middle, as shown in the image at right. 
*It goes from one at the ends to zero in the middle.
*It goes from 1 at the start of the visible spectrum to 0 in some point in the middle until its end.
*It goes from 0 at the start of the visible spectrum to 1 at some point in the middle until its end. 

The first type produces colors that are similar to the [[spectral colors]] and follow roughly the horseshoe-shaped portion of the [[Chromaticity diagram#The CIE xy chromaticity diagram|CIE xy chromaticity diagram]] (the [[spectral locus]]), but are, in surfaces, more [[colorfulness|chromatic]], although less [[Visible spectrum|spectrally]] pure. The second type produces colors that are similar to (but, in surfaces, more chromatic and less spectrally pure than) the colors on the straight line in the CIE xy chromaticity diagram (the [[line of purples]]), leading to [[magenta]] or purple-like colors. The third type produces the colors located in the "warm" sharp edge of the optimal color solid (this will be explained later in the article). The fourth type produces the colors located in the "cold" sharp edge of the optimal color solid.

In optimal color solids, the colors of the visible spectrum are theoretically black, because their reflectance spectrum is 1 (100%) in only one wavelength, and 0 in all of the other infinite visible wavelengths that there are, meaning that they have a lightness of 0 with respect to white, and will also have 0 chroma, but, of course, 100% of spectral purity. In short: In optimal color solids, spectral colors are equivalent to black (0 lightness, 0 chroma), but have full spectral purity (they are located in the horseshoe-shaped spectral locus of the chromaticity diagram).<ref name="thi"/>

In linear color spaces, such as [[LMS color space|LMS]] or [[CIE 1931 color space|CIE 1931 XYZ]], the set of [[Ray (geometry)|rays]] that start at the origin (black, (0, 0, 0)) and pass through all the points that represent the colors of the visible spectrum, and the portion of a plane that passes through the violet half-line and the red half-line (both ends of the visible spectrum), generate the "spectrum cone". The black point (coordinates (0, 0, 0)) of the optimal color solid (and only the black point) is tangent to the "spectrum cone", and the white point ((1, 1, 1)) (only the white point) is tangent to the "inverted spectrum cone", with the "inverted spectrum cone" being [[Symmetry|symmetrical]] to the "spectrum cone" with respect to the middle gray point ((0.5, 0.5, 0.5)). This means that, in linear color spaces, the optimal color solid is centrally symmetric.<ref name="thi"/>

[[File:Visible gamut within CIEXYZ color space D65 whitepoint mesh.webm|thumb|185x185px|Optimal color solid or Rösch–MacAdam color solid (with [[Illuminant D65|D65]] [[white point]]) plotted within [[CIE 1931 color space|CIE 1931 XYZ color space]]. Notice the central symmetry of the solid, and the two sharp edges, one with warm colors and the other one with cold colors.]]

In most color spaces, the surface of the optimal color solid is smooth, except for two points (black and white); and two sharp edges: the "[[Heat|warm]]" edge, which goes from black, to red, to orange, to yellow, to white; and the "cold" edge, which goes from black, to deep [[Violet (color)|violet]], to blue, to [[cyan]], to white. This is due to the following: If the portion of the reflectance spectrum of a color is spectral red (which is located at one end of the spectrum), it will be seen as black. If the size of the portion of total reflectance is increased, now covering from the red end of the spectrum to the yellow wavelengths, it will be seen as red or orange. If the portion is expanded even more, covering some green wavelengths, it will be seen as yellow. If it is expanded even more, it will cover more wavelengths than the yellow [[Color solid#Maximum chroma colors, semichromes, or full colors|semichrome]] does, approaching white, until it is reached when the full spectrum is reflected. The described process is called "cumulation". Cumulation can be started at either end of the visible spectrum (we just described cumulation starting from the red end of the spectrum, generating the "warm" sharp edge), cumulation starting at the violet end of the spectrum will generate the "cold" sharp edge.<ref name="thi"/>

[[File:Visible gamut within CIELUV color space D65 whitepoint mesh.webm|thumb|Optimal color solid plotted within the [[CIELUV color space|CIE L* u* v* color space]], with [[Illuminant D65|D65]] [[white point]]. Notice that it has two sharp edges, one with warm colors, and the other one with cold colors.|200x200px]]

On modern computers, it is possible to calculate an optimal color solid with great precision in seconds. Usually, only the MacAdam limits (the optimal colors, the boundary of the Optimal color solid) are computed, because all the other (non-optimal) possible surface colors exist inside the boundary.

[[File:Optimal-color-solid,FL4,XYZ.gif|right|thumb|185x185px|MacAdam limits for illuminant [[Standard illuminant#Illuminant series F|CIE F4]] in [[CIE 1931 color space#CIE xyY color space|CIE xyY color space]]]]

=== Maximum chroma colors, semichromes, or full colors ===
Each hue has a maximum chroma point, semichrome, or full color; objects cannot have a color of that hue with a higher chroma. They are the most chromatic, vibrant colors that objects can have. They were called '''semichromes''' or '''full colors''' by the German chemist and philosopher [[Wilhelm Ostwald]] in the early 20th century.<ref name="thi"/><ref name="Ost">{{cite journal |last1=Liberini |first1=Simone |last2=Rizzi |first2=Alessandro |year=2023 |title=Munsell and Ostwald colour spaces: A comparison in the field of hair colouring |url=https://onlinelibrary.wiley.com/doi/10.1002/col.22818|journal=Color Research and Application|volume=48 |pages=6–20 |doi=10.1002/col.22818 |hdl=2434/940227 |hdl-access=free }}</ref>

If B is the complementary wavelength of wavelength A, then the straight line that connects A and B passes through the achromatic axis in a linear color space, such as LMS or CIE 1931 XYZ. If the reflectance spectrum of a color is 1 (100%) for all the wavelengths between A and B, and 0 for all the wavelengths of the other [[half]] of the color space, then that color is a maximum chroma color, semichrome, or full color (this is the explanation to why they were called '''semi'''chromes). Thus, maximum chroma colors are a type of optimal color.<ref name="thi"/><ref name="Ost"/>

As explained, full colors are far from being monochromatic (physically, not perceptually). If the spectral purity of a semichrome is increased, its [[colorfulness|chroma]] decreases, because it will approach the visible spectrum, ergo, it will approach black.<ref name="thi"/>

In perceptually uniform color spaces, the lightness of the full colors varies from around 30% in the violetish blue hues, to around 90% in the [[yellow]]ish hues. The chroma of each maximum chroma point also varies depending on the hue; in optimal color solids plotted in perceptually uniform color spaces, semichromes like red, green, violet, and [[magenta]] have a high chroma, while semichromes like yellow, orange, and [[cyan]] have a slightly lower chroma.

[[File:Munsell 5 PB 5Y.png|thumb|Slice of the Munsell color space in the hues of 5PB and 5Y. The point farthest from the achromatic axis in each of these two hue slices is the maximum chroma color, semichrome, or full color of that hue|200x200px]]

== Cultural perspective ==

The meanings and associations of colors can play a major role in works of art, including literature.<ref name="Westfahl2005">{{cite book |first=Gary |last=Westfahl |url=https://books.google.com/books?id=SQMQQyIaACYC&pg=PA142 |title=The Greenwood Encyclopedia of Science Fiction and Fantasy: Themes, Works, and Wonders |publisher=Greenwood Publishing Group |year=2005 |isbn=978-0313329517 |pages=142–143}}</ref>

=== Associations ===
Individual colors have a variety of cultural associations such as [[national colors]] (in general described in individual color articles and [[color symbolism]]). The field of [[color psychology]] attempts to identify the effects of color on human emotion and activity. [[Chromotherapy]] is a pseudoscientific therapy attributed to various Eastern traditions. Colors have different associations in different countries and cultures.<ref>{{cite web |title=Chart: Color Meanings by Culture |url=http://www.globalization-group.com/edge/resources/color-meanings-by-culture/ |url-status=dead |archive-url=http://webarchive.loc.gov/all/20101012003744/http://www.globalization-group.com/edge/resources/color-meanings-by-culture/ |archive-date=2010-10-12 |access-date=2010-06-29}}</ref>

Different colors have been demonstrated to have effects on cognition. For example, researchers at the University of Linz in Austria demonstrated that the color red significantly decreases cognitive functioning in men.<ref>{{cite journal |last1=Dzulkifli |first1=Mariam |last2=Mustafar |first2=Muhammad |year=2013 |title=The Influence of Colour on Memory Performance: A Review |journal=The Malaysian Journal of Medical Sciences |volume=20 |issue=2 |pages=3–9 |doi=10.1016/j.chb.2010.06.010|s2cid=17764339 }}</ref> The combination of the colors red and yellow together can induce hunger, which has been capitalized on by a number of chain restaurants.<ref>{{cite web |title=There's a sneaky reason why you always see red and yellow on fast food logos |url=https://www.businessinsider.com/fast-food-colors-make-you-hungry-2018-9?r=MX&IR=T#:~:text=There's%20a%20sneaky%20reason%20why,yellow%20on%20fast%20food%20logos&text=Fast%20food%20chains%2C%20from%20McDonald's,feel%20more%20hungry%20and%20impulsive. |access-date=2022-02-09 |website=[[Business Insider]]}}</ref>

Color plays a role in memory development too. A photograph that is in black and white is slightly less memorable than one in color.<ref>{{cite journal |last1=Gnambs |first1=Timo |last2=Appel |first2=Markus |last3=Batinic |first3=Bernad |year=2010 |title=Color red in web-based knowledge testing |journal=Computers in Human Behavior |volume=26 |issue=6 |pages=1625–1631 |doi=10.1016/j.chb.2010.06.010|s2cid=17764339 }}</ref> Studies also show that wearing bright colors makes one more memorable to people they meet.

=== Terminology ===
{{main|Color term}}
{{see also|Lists of colors|Web colors}}
Colors vary in several different ways, including [[hue]] (shades of [[red]], [[orange (colour)|orange]], [[yellow]], [[green]], [[blue]], and [[violet (color)|violet]], etc.), [[colorfulness|saturation]], [[brightness]]. Some color words are derived from the name of an object of that color, such as "[[orange (colour)|orange]]" or "[[salmon (color)|salmon]]", while others are abstract, like "red".

In the 1969 study ''[[Basic Color Terms]]: Their Universality and Evolution'', [[Brent Berlin]] and [[Paul Kay]] describe a pattern in naming "basic" colors (like "red" but not "red-orange" or "dark red" or "blood red", which are "shades" of red). All languages that have two "basic" color names distinguish dark/cool colors from bright/warm colors. The next colors to be distinguished are usually red and then yellow or green. All languages with six "basic" colors include black, white, red, green, blue, and yellow. The pattern holds up to a set of twelve: black, gray, white, pink, red, orange, yellow, green, blue, purple, brown, and [[azure (color)|azure]] (distinct from blue in [[Russian language|Russian]] and [[Italian language|Italian]], but not English).

== See also ==

* [[Chromophore]]
* [[Color analysis]]
* [[Color in Chinese culture]]
* [[Color mapping]]
* [[Complementary colors]]
* [[Impossible color]]
* [[International Color Consortium]]
* [[International Commission on Illumination]]
* [[Lists of colors]] [[list of colors (compact)|(compact version)]]
* [[Neutral color]]
* [[Pearlescent coating]] including Metal effect pigments
* [[Pseudocolor]]
* [[Primary color|Primary]], [[secondary color|secondary]] and [[tertiary color]]s

== References ==

{{reflist}}

==External links==
* {{Britannica|id=126658|title=Color}}
* {{cite SEP|url-id=color|title=Color|last=Maund|first=Barry}}
* {{cite IEP|url-id=color|title=Color}}

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