Editing Color (section)

== 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.