Editing Red (section)

== In science and nature ==

=== Seeing red ===
[[File:Madrid Bullfight.JPG|thumb|Bulls, like dogs and many other animals, have [[dichromacy]], which means they cannot distinguish the color red. They charge the matador's cape because of its motion, not its color.]]
The human eye sees red when it looks at light with a wavelength between approximately 625 and 740 [[Nanometre|nanometers]].<ref name="Georgia State University Department of Physics and Astronomy" /> It is a primary color in the [[RGB color model]] and the light just past this range is called infrared, or below red, and cannot be seen by human eyes, although it can be sensed as heat.<ref>{{cite web |title=What Wavelength Goes With a Color? |website=Atmospheric Science Data Center |url=http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |access-date=2009-04-15 |url-status=dead |archive-url=https://web.archive.org/web/20110720105431/http://science-edu.larc.nasa.gov/EDDOCS/Wavelengths_for_Colors.html |archive-date=2011-07-20}}</ref> In the language of optics, red is the color evoked by light that stimulates neither the S or the M (short and medium wavelength) cone cells of the retina, combined with a fading stimulation of the L (long-wavelength) cone cells.<ref>{{Cite book |title=Introduction to psychology|last=Kalat|first=J. W. |date=2005|publisher=Thomson/Wadsworth|isbn=978-0534624606 |edition=7th|location=Belmont, CA|oclc=56799330|pages=[https://archive.org/details/isbn_053462460x/page/105 105] |url=https://archive.org/details/isbn_053462460x/page/105}}</ref>

[[Primate]]s can distinguish the full range of the colors of the spectrum visible to humans, but many kinds of mammals, such as dogs and cattle, have [[dichromacy]], which means they can see blues and yellows, but cannot distinguish red and green (both are seen as gray). Bulls, for instance, cannot see the red color of the cape of a bullfighter, but they are agitated by its movement.<ref name="Ali&Klyne1985">{{Cite book |last1=Ali |first1=Mohamed Ather |last2=Klyne |first2=M.A. |title=Vision in Vertebrates |place=New York |publisher=Plenum Press |year=1985 |pages=174–75 |isbn=978-0-306-42065-8}}</ref> (See [[color vision]]).

One theory for why primates developed sensitivity to red is that it allowed ripe fruit to be distinguished from unripe fruit and inedible vegetation.<ref>{{cite web |url=http://anthro.palomar.edu/primate/color.htm |title=Primate Color Vision |last=O'Neil |first=Dennis |date=March 19, 2010 |website=Primates |publisher=Palomar Community College |access-date=22 April 2010 |location=San Marcos, California |archive-url=https://web.archive.org/web/20110720024858/http://anthro.palomar.edu/primate/color.htm |archive-date=20 July 2011 |url-status=live }}</ref> This may have driven further adaptations by species taking advantage of this new ability, such as the emergence of red faces.<ref name="scidaily">{{cite web|url=https://www.sciencedaily.com/releases/2007/05/070524155313.htm|title=Color Vision Drove Primates To Develop Red Skin And Hair, Study Finds|last1=Hogan|first1=Dan|last2=Hogan|first2=Michele|date=May 25, 2007|website=Science News|publisher=ScienceDaily|location=Rockville, Maryland|access-date=22 April 2010|archive-url=https://web.archive.org/web/20100507093408/http://www.sciencedaily.com/releases/2007/05/070524155313.htm|archive-date=7 May 2010|url-status=live}}</ref>

Red light is used to help adapt [[adaptation (eye)|night vision]] in low-light or night time, as the [[rod cell]]s in the human eye are not sensitive to red.<ref>{{cite web|url=http://www.olympusmicro.com/primer/lightandcolor/humanvisionintro.html|title=Human Vision and Color Perception|website=Olympus Microscopy Resource Center|archive-url=https://web.archive.org/web/20170705094711/http://www.olympusmicro.com/primer/lightandcolor/humanvisionintro.html|archive-date=July 5, 2017|url-status=dead|access-date=Nov 23, 2018}}</ref><ref>{{cite web |url=https://www.pbs.org/wgbh/nova/worlds/stargazer.html#Sensitize%20Your%20Eyes |title=Be a Stargazer |access-date=2007-09-25 |website=Sensitize Your Eyes |archive-url=https://web.archive.org/web/20071012003807/http://www.pbs.org/wgbh/nova/worlds/stargazer.html#Sensitize%20Your%20Eyes |archive-date=2007-10-12 |url-status=live }}</ref>

=== In color theory and on a computer screen ===
In the [[RYB color model]], which is the basis of [[traditional color theory]], red is one of the three [[primary color]]s, along with blue and yellow. Painters in the Renaissance mixed red and blue to make violet:  [[Cennino Cennini]], in his 15th-century manual on painting, wrote, "If you want to make a lovely violet colour, take fine lac ([[Lake pigments#History and art|red lake]]), [[ultramarine blue]] (the same amount of the one as of the other) with a binder"; he noted that it could also be made by mixing blue [[indigo]] and red [[hematite]].<ref>{{Cite book|url=https://archetype.co.uk/our-titles/cennino-cenninis-i-il-libro-dellarte-i/?id=224|title=Cennino Cennini's Il libro dell'arte: a new English translation and commentary with Italian transcription|publisher=Archetype|year=2015|isbn=9781909492288|editor-last=Broecke|editor-first=Lara|location=London|pages=115|oclc=910400601|access-date=2018-11-23|archive-date=2020-11-01|archive-url=https://web.archive.org/web/20201101102156/https://archetype.co.uk/our-titles/cennino-cenninis-i-il-libro-dellarte-i/?id=224|url-status=live}}</ref>

In the CMY and [[CMYK color model|CMYK]] color models, red is a secondary color subtractively mixed from magenta and yellow.{{citation needed|date=January 2023}}

In the [[RGB color model]], red, green and blue are [[additive primary colors]]. Red, green and blue light combined makes white light, and these three colors, combined in different mixtures, can produce nearly any other color. This principle is used to generate colors on such as computer monitors and televisions.  For example, magenta on a computer screen is made by a similar formula to that used by Cennino Cennini in the Renaissance to make violet, but using [[additive color]]s and light instead of pigment:  it is created by combining red and blue light at equal intensity on a black screen. Violet is made on a computer screen in a similar way, but with a greater amount of blue light and less red light.<ref name="huevaluechroma1">{{Cite web|url=http://www.huevaluechroma.com/041.php|title=The Dimensions of Colour: Part 4. Additive Mixing|last=Briggs|first=David|website=huevaluechroma.com|access-date=Nov 23, 2018|archive-url=https://web.archive.org/web/20181113180931/http://www.huevaluechroma.com/041.php|archive-date=November 13, 2018|url-status=live}}</ref>

<gallery mode="packed" heights="150">
File:Boutet 1708 color circles.jpg|In a traditional [[color wheel]] from 1708, red, yellow and blue are primary colors. Red and yellow make orange; red and blue make violet.
File:RGB combination on wall.png|In modern color theory, red, green and blue are the additive primary colors, and together they make white. A combination of red, green and blue light in varying proportions makes all the colors on your computer screen and television screen.
File:RGB pixels.jpg|Tiny red, green and blue [[sub-pixel]]s (enlarged on left side of image) create the colors you see on your computer screen and TV. 
</gallery>

=== Color of sunset ===
{{main|Sunset#Colors}}
[[File:Crimson sunset.jpg|thumb|right|Sunsets and sunrises are often red because of an optical effect called [[Rayleigh scattering]].]]
As a ray of white sunlight travels through the atmosphere to the eye, some of the colors are scattered out of the beam by air molecules and [[Atmospheric particulate matter|airborne particles]] due to [[Rayleigh scattering]], changing the final color of the beam that is seen. Colors with a shorter wavelength, such as blue and green, scatter more strongly, and are removed from the light that finally reaches the eye.<ref name="saha">{{cite book |author=K. Saha |title=The Earth's Atmosphere – Its Physics and Dynamics |year=2008 |publisher=Springer |isbn=978-3-540-78426-5 |page=107}}</ref> At [[sunrise]] and [[sunset]], when the path of the sunlight through the atmosphere to the eye is longest, the blue and green components are removed almost completely, leaving the longer wavelength orange and red light. The remaining reddened sunlight can also be scattered by cloud droplets and other relatively large particles, which give the sky above the horizon its red glow.<ref name="guenther">{{cite book|editor-last=Guenther |editor-first=B.|title=Encyclopedia of Modern Optics |publisher=Elsevier|year=2005|isbn=9780123693952|volume=1 |pages=186}}</ref>

=== Lasers ===
[[Laser]]s emitting in the red region of the spectrum have been available since the invention of the [[ruby laser]] in 1960. In 1962 the red [[helium–neon laser]] was invented,<ref>{{Cite journal|last1=White|first1=A. D.|last2=Rigden |first2=J. D.|date=1962 |title=Continuous Gas Maser Operation in the Visible|journal=Proc. IRE|volume=50 |pages=1697}} US Patent 3242439.</ref> and these two types of lasers were widely used in many scientific applications including [[holography]], and in education. Red helium–neon lasers were used commercially in [[LaserDisc]] players. The use of red [[laser diode]]s became widespread with the commercial success of modern [[DVD]] players, which use a 660&nbsp;nm laser diode technology. Today, red and red-orange laser diodes are widely available to the public in the form of extremely inexpensive [[laser pointer]]s. Portable, high-powered versions are also available for various applications.<ref name="laserglow">{{cite web |url=http://www.laserglow.com/GRH|title=Laserglow – Blue, Red, Yellow, Green Lasers|website=Laserglow.com|archive-url=https://web.archive.org/web/20110805054301/https://www.laserglow.com/GRH|archive-date=August 5, 2011|url-status=dead|access-date=Sep 20, 2011}}</ref> More recently, 671&nbsp;nm diode-pumped solid state ([[DPSS]]) lasers have been introduced to the market for all-DPSS laser display systems, [[particle image velocimetry]], [[Raman spectroscopy]], and holography.<ref name="laserglow2">{{cite web |url=http://www.laserglow.com/GRH |title=Laserglow&nbsp;– Lab/OEM Lasers |publisher=Laserglow.com |access-date=2011-09-20 |archive-url=https://web.archive.org/web/20110915034320/http://www.laserglow.com/GRH |archive-date=2011-09-15 |url-status=live }}</ref>

Red's wavelength has been an important factor in laser technologies; red lasers, used in early [[compact disc]] technologies, are being replaced by blue lasers, as red's longer wavelength causes the laser's recordings to take up more space on the disc than would blue-laser recordings.<ref>{{Cite web|url=http://www.usbyte.com/common/dvd_7.htm |access-date=Nov 23, 2018|title=DVD |website=usbyte.com |publisher=eMag Solutions LLC|url-status=dead|archive-url= https://web.archive.org/web/20100106063624/http://www.usbyte.com/common/dvd_7.htm|archive-date=January 6, 2010}}</ref>

=== Astronomy ===
* [[Mars]] is called the Red Planet because of the reddish color imparted to its surface by the abundant [[iron oxide]] present there.<ref>{{cite web |url=http://www.cs.ucf.edu/~MidLink/Mars.html|title=Mars, The Red Planet|last1=Adams|first1=Melanie |last2=Raynor |first2=Natasha|date=Sep 19, 1994 – Mar 12, 2009 |website=MidLink Magazine|publisher=North Carolina State University |archive-url=https://web.archive.org/web/20070712223543/http://www.cs.ucf.edu/~MidLink/Mars.html|archive-date=July 12, 2007|url-status=dead|access-date=12 April 2010}}</ref>
* Astronomical objects that are moving away from the observer exhibit a Doppler [[redshift|red shift]].
* [[Jupiter]]'s surface displays a [[Great Red Spot]] caused by an oval-shaped mega storm south of the planet's [[equator]].<ref>{{cite web|title=The Great Red Spot |url=http://csep10.phys.utk.edu/astr161/lect/jupiter/redspot.html|last1=Cardall |first1=Christian|last2=Daunt |first2=Steven|year=2003|website=The Solar System |publisher=University of Tennessee|url-status=dead|archive-url=https://web.archive.org/web/20100331125637/http://csep10.phys.utk.edu/astr161/lect/jupiter/redspot.html |archive-date=March 31, 2010|access-date=Apr 12, 2010}}</ref>
* [[Red giant]]s are stars that have exhausted the supply of [[hydrogen]] in their cores and switched to [[thermonuclear fusion]] of hydrogen in a shell that surrounds its core. They have radii tens to hundreds of times larger than that of the [[Sun]]. However, their outer envelope is much lower in temperature, giving them an orange hue. Despite the lower energy density of their envelope, red giants are many times more luminous than the Sun due to their large size.
* [[Red supergiant]]s like [[Betelgeuse]], [[Antares]], [[Mu Cephei]], [[VV Cephei]], and [[VY Canis Majoris]] one of the [[List of largest known stars|biggest stars]] in the [[Universe]], are the biggest variety of red giants. They are huge in size, with radii 200 to 1700 times greater than the Sun, but relatively cool in temperature (3000–4500 K), causing their distinct red tint.
* A [[red dwarf]] is a small and relatively [[temperature|cool]] [[star]], which has a mass of less than half that of the [[Sun]] and a surface temperature of less than 4,000&nbsp;[[kelvin|K]]. Red dwarfs are by far the most common type of star in the Galaxy, but due to their low luminosity, from Earth, none are visible to the naked eye.<ref>{{Cite web|title=The Brightest Red Dwarf |last=Croswell|first=Ken|website=kencroswell.com |url=http://kencroswell.com/thebrightestreddwarf.html |access-date=Nov 23, 2018 |archive-url=https://web.archive.org/web/20181020223954/http://www.kencroswell.com/thebrightestreddwarf.html|archive-date=October 20, 2018|url-status=live}}</ref>
* [[Interstellar reddening]] is caused by the extinction of radiation by dust and gas

<gallery mode="packed" heights="150px">
Mars atmosphere 2.jpg|[[Mars]] appears to be red because of [[iron oxide]] on its surface.
Mira 1997.jpg|[[Mira]], a [[red giant]]
File:RedDwarfNASA.jpg|Artist's impression of a [[red dwarf]], a small, relatively cool star that appears red due to its temperature
</gallery>

=== Pigments and dyes ===
{{See also|Red pigments}}
<gallery mode="packed" heights="150px">
Roussillon sentier des ocres2.JPG|Red [[ochre]] cliffs near [[Roussillon]] in France. Red ochre is composed of clay tinted with hematite. Ochre was the first pigment used by man in prehistoric cave paintings.
Vermillon pigment.jpg|Vermilion pigment, made from cinnabar. This was the pigment used in the murals of [[Pompeii]] and to color Chinese [[lacquerware]] beginning in the [[Song dynasty]].
Red lead.jpg|[[Red lead]], also known as [[minium (pigment)|minium]], has been used since the time of the ancient Greeks. Chemically it is known as [[lead tetroxide]]. The Romans prepared it by the roasting of lead white pigment. It was commonly used in the Middle Ages for the headings and decoration of illuminated manuscripts.
Rubia tinctorum - Köhler–s Medizinal-Pflanzen-123.jpg|The roots of the ''[[Rubia tinctorum]]'', or madder plant, produced the most common red dye used from ancient times until the 19th century.
Alizarin-sample.jpg|[[Alizarin]] was the first synthetic red dye, created by German chemists in 1868. It duplicated the colorant in the madder plant, but was cheaper and longer lasting. After its introduction, the production of natural dyes from the madder plant virtually ceased.
</gallery>

=== Food coloring ===
The most common synthetic food coloring today is [[Allura Red AC]], a red [[azo dye]] that goes by several names including: '''Allura Red''', '''Food Red 17''', '''C.I. 16035''', '''FD&C Red 40''',<ref name="Shampoo">{{cite web |url=http://www.cfsan.fda.gov/~dms/col-221.html |title=From Shampoo to Cereal: Seeing to the Safety of Color Additives |access-date=2008-06-04 |url-status=dead |archive-url=https://web.archive.org/web/20080115194446/http://www.cfsan.fda.gov/~dms/col-221.html |archive-date=January 15, 2008 }}</ref><ref name="Food Colour Facts">{{cite web |url=http://www.cfsan.fda.gov/~lrd/colorfac.html|title=Food Color Facts|date=January 1993|url-status=dead|archive-url=https://web.archive.org/web/20071001050645/http://www.cfsan.fda.gov/~lrd/colorfac.html|archive-date=October 1, 2007 |access-date=2006-08-18}}</ref> It was originally manufactured from coal tar, but now is mostly made from petroleum.<ref>{{cite web |title=E129 – Allura Red AC |url=https://proe.info/additives/e129 |website=proe.info |access-date=2022-10-31 |archive-date=2022-10-31 |archive-url=https://web.archive.org/web/20221031125505/https://proe.info/additives/e129 |url-status=live }}</ref>

In Europe, Allura Red AC is not recommended for consumption by children. It is banned in Denmark, Belgium, France and Switzerland, and was also banned in Sweden until the country joined the European Union in 1994.<ref name="Food Colour Facts"/> The [[European Union]] approves Allura Red AC as a food colorant, but EU countries' local laws banning food colorants are preserved.<ref>{{Cite journal|date=Jun 30, 1994|title=European Parliament and Council Directive 94/36/EC on colours for use in foodstuffs |journal=European Parliament and Council of the European Union|via=EUR-Lex |url=http://data.europa.eu/eli/dir/1994/36/oj|access-date=November 23, 2018|url-status=live|archive-url=https://web.archive.org/web/20201101102147/https://eur-lex.europa.eu/eli/dir/1994/36/oj|archive-date=November 1, 2020}} Precise volume, tome, and page numbers for all languages are available on the cited website.</ref>

In the United States, Allura Red AC is approved by the [[Food and Drug Administration]] (FDA) for use in [[cosmetics]], [[drug]]s, and food. It is used in some tattoo inks and is used in many products, such as [[soft drink]]s, children's medications, and [[cotton candy]]. On June 30, 2010, the [[Center for Science in the Public Interest]] (CSPI) called for the FDA to ban Red 40.<ref>{{Cite news|title=Group urges ban of 3 common dyes |url=http://thechart.blogs.cnn.com/2010/06/30/food-dyes-a-health-risk/ |last=Young|first=Saundra|date=Jun 30, 2010 |work=CNN|language=en|access-date=Jul 1, 2010|archive-url=https://web.archive.org/web/20100703013023/http://pagingdrgupta.blogs.cnn.com/2010/06/30/food-dyes-a-health-risk/ |archive-date=July 3, 2010|url-status=dead}}</ref> [[Red 3]] dye was banned in the United States in 2025.<ref>{{Cite web |last=Rogers |first=Kristen |date=2025-01-17 |title=Red dye No. 3 is now banned in the US. Here’s what studies show about more common dyes |url=https://www.cnn.com/2025/01/17/health/red-40-food-dyes-wellness/index.html |access-date=2025-02-24 |website=CNN |language=en}}</ref>

Because of public concerns about possible health risks associated with synthetic dyes, many companies have switched to using natural pigments such as [[carmine]], made from crushing the tiny female [[cochineal]] insect. This insect, originating in Mexico and Central America, was used to make the brilliant [[Scarlet (color)|scarlet]] dyes of the European Renaissance.{{Citation needed|date=July 2021}}

=== Autumn leaves ===
{{More citations needed|section|date=July 2021}}
The red of autumn leaves is produced by pigments called [[anthocyanin]]s. They are not present in the leaf throughout the growing season, but are actively produced towards the end of summer.<ref name="tree1044">{{cite journal|last1=Archetti|first1=Marco|last2=Döring|first2=T. F.|last3=Hagen|first3=S. B.|last4=Hughes|first4=N. M.|last5=Leather|first5=S. R.|last6=Lee|first6=D. W.|last7=Lev-Yadun|first7=S.|last8=Manetas|first8=Y.|last9=Ougham|first9=H. J.|display-authors=3|year=2011|title=Unravelling the evolution of autumn colours: an interdisciplinary approach|journal=Trends in Ecology & Evolution|volume=24|issue=3|pages=166–73|doi=10.1016/j.tree.2008.10.006|pmid=19178979|last10=Schaberg|first10=Paul G.|last11=Thomas|first11=Howard}}</ref> They develop in late summer in the [[sap]] of the cells of the leaf, and this development is the result of complex interactions of many influences—both inside and outside the plant. Their formation depends on the breakdown of sugars in the presence of bright light as the level of [[phosphate]] in the leaf is reduced.<ref>{{Cite book|title=Plant pigments and their manipulation|date=2004|publisher=[[Wiley-Blackwell]]|isbn=978-1405117371|editor-last=Davies|editor-first=Kevin M.|location=Oxford|pages=6|oclc=56963804}}</ref>

During the summer growing season, phosphate is at a high level. It has a vital role in the breakdown of the [[sugar]]s manufactured by chlorophyll. But in the fall, phosphate, along with the other chemicals and nutrients, moves out of the leaf into the [[Plant stem|stem]] of the plant. When this happens, the sugar-breakdown process changes, leading to the production of anthocyanin pigments. The brighter the light during this period, the greater the production of anthocyanins and the more brilliant the resulting color display. When the days of autumn are bright and cool, and the nights are chilly but not freezing, the brightest colorations usually develop.

Anthocyanins temporarily color the edges of some of the very young [[leaf|leaves]] as they unfold from the [[bud]]s in early spring. They also give the familiar color to such common fruits as [[cranberry|cranberries]], [[Red Delicious|red apples]], [[blueberry|blueberries]], [[cherry|cherries]], [[raspberry|raspberries]], and [[plum]]s.

Anthocyanins are present in about 10% of tree species in temperate regions, although in certain areas—a [[autumn in New England|famous example being New England]]—up to 70% of tree species may produce the pigment.<ref name="tree1044" /> In autumn forests they appear vivid in the [[maple]]s, [[oak]]s, [[sourwood]], [[Liquidambar|sweetgums]], [[dogwood]]s, [[Tupelo (tree)|tupelo]]s, [[cherry]] trees and [[persimmon]]s. These same pigments often combine with the carotenoids' colors to create the deeper orange, fiery reds, and bronzes typical of many hardwood species. (See [[Autumn leaf color]]).

=== Blood and other reds in nature ===
{{More citations needed|section|date=July 2021}}
Oxygenated blood is red due to the presence of oxygenated [[hemoglobin]] that contains iron molecules, with the iron components reflecting red light.<ref name="UCSBblood">{{cite web |title=Why is blood red? |url=http://scienceline.ucsb.edu/getkey.php?key=2419 |website=[[University of California, Santa Barbara]] |access-date=3 October 2015 |archive-url=https://web.archive.org/web/20150920120837/http://scienceline.ucsb.edu/getkey.php?key=2419 |archive-date=20 September 2015 |url-status=dead }}</ref><ref>{{cite web|url=http://www.medicinenet.com/hemoglobin/article.htm|title=Hemoglobin|last=Nabili|first=Siamak|website=Procedures and Tests|publisher=MedicineNet|page=1|archive-url=https://web.archive.org/web/20100323145336/http://www.medicinenet.com/hemoglobin/article.htm|archive-date=March 23, 2010|url-status=dead|access-date=Apr 12, 2010}}</ref> Red meat gets its color from the iron found in the [[myoglobin]] and hemoglobin in the muscles and residual blood.<ref>{{Cite journal|last1=Fleming|first1=H. P.|last2=Blumer|first2=T. N.|last3=Craig|first3=H. B.|date=1960-11-01|title=Quantitative Estimations of Myoglobin and Hemoglobin in Beef Muscle Extracts|journal=Journal of Animal Science|language=en|volume=19|issue=4|pages=1164–1171|doi=10.2527/jas1960.1941164x|issn=0021-8812|via=[[WorldCat]]}}</ref>

Plants like [[apple]]s, [[Garden strawberry|strawberries]], [[cherry|cherries]], [[tomato]]es, [[Capsicum|peppers]], and [[pomegranate]]s are often colored by forms of [[carotenoid]]s, red pigments that also assist [[photosynthesis]].<ref>{{cite web |url=http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html |title=Photosynthetic Pigments |last=Speer |first=Brian |website=UCMP Glossary |publisher=University of California Museum of Paleontology |access-date=22 April 2010 |location=University of California |archive-url=https://web.archive.org/web/20100616181829/http://www.ucmp.berkeley.edu/glossary/gloss3/pigments.html |archive-date=16 June 2010 |url-status=live }}</ref>

<gallery mode="packed" heights="150px">
Agarplate redbloodcells edit.jpg|Red blood cell [[agar plate|agar]]. Blood appears red due to the iron molecules in blood cells.
Can Setter dog GFDL.jpg|A [[red setter]] or Irish setter
Red Fox (Vulpes vulpes) -British Wildlife Centre-8.jpg|A pair of European [[red fox]]es
Erithacus-rubecula-melophilus Dublin-Ireland.jpg|The [[European robin]] or robin redbreast
</gallery>

=== Hair color ===
{{Main|Red hair}}
[[File:Woman redhead natural portrait 1.jpg|thumb|Red hair only occurs in 1–2% of the human population.]]
Red hair occurs naturally on approximately 1–2% of the human population.<ref>{{Cite web|url=http://www.garreau.com/main.cfm?action=chapters&id=20|title=Red Alert!|last=Garreau|first=Joel|date=Mar 18, 2002|website=The Garreau Group|publisher=The Washington Post|archive-url=https://web.archive.org/web/20130527035304/http://www.garreau.com/main.cfm?action=chapters&id=20|archive-date=May 27, 2013|url-status=dead|access-date=Nov 23, 2018}}</ref> It occurs more frequently (2–6%) in people of northern or western European ancestry, and less frequently in other populations. Red hair appears in people with two copies of a [[Dominance relationship|recessive gene]] on [[chromosome 16]] which causes a mutation in the [[melanocortin 1 receptor|MC1R]] protein.<ref name="thetech">{{cite web |last=Starr |first=D. Barry |url=https://www.thetech.org/ask-a-geneticist/articles/2004/ask44/ |title=Neither my husband nor I have redheads in our family. How did our child get red hair? |date=26 August 2004 |website=[[The Tech Interactive]] |series=Ask a Geneticist |access-date=4 August 2024 |quote=When someone has both of their MC1R genes mutated, this conversion doesn't happen anymore and you get a buildup of pheomelanin, which results in red hair}}</ref>

Red hair varies from a deep [[Burgundy (color)|burgundy]] through [[Orange (color)#Burnt orange|burnt orange]] to bright [[Copper (color)|copper]]. It is characterized by high levels of the reddish pigment [[pheomelanin]] (which also accounts for the red color of the lips) and relatively low levels of the dark pigment [[eumelanin]]. The term "redhead" (originally ''redd hede'') has been in use since at least 1510.<ref>{{cite web|title=redhead, n. and adj.|work=OED Online|url=http://oed.com/view/Entry/160309|publisher=[[Oxford University Press]]|access-date=7 August 2011|date=June 2011|archive-date=1 November 2020|archive-url=https://web.archive.org/web/20201101102147/https://oed.com/start;jsessionid=A69DA6EA6221596E8BC455D7F3F021F3?authRejection=true&url=%2Fview%2FEntry%2F160309|url-status=live}}</ref>

=== In animal and human behavior ===
Red is associated with [[Dominance hierarchy|dominance]] in a number of animal species.<ref name="Little & Hill 2007">{{cite journal|last1=Little|first1=A. C.|last2=Hill|first2=R. A.|year=2007|title=Attribution to red suggests special role in dominance signalling|journal=Journal of Evolutionary Psychology|volume=5|issue=1|pages=161–168|doi=10.1556/JEP.2007.1008|doi-access=free}}</ref> For example, in [[mandrill]]s, red coloration of the face is greatest in [[Dominance hierarchy|alpha]] males, increasingly less prominent in lower ranking subordinates, and directly correlated with levels of [[testosterone]].<ref>{{Cite journal |last1=Setchell |first1=J. |last2=Smith |first2=T. |last3=Wickings |first3=E. |last4=Knapp |first4=L. |title=Social correlates of testosterone and ornamentation in male mandrills |journal=Hormones and Behavior |volume=54 |issue=3 |pages=365–72 |year=2008 |pmid=18582885 |doi=10.1016/j.yhbeh.2008.05.004 |s2cid=28843140 |url=http://dro.dur.ac.uk/5092/1/5092.pdf?DDD5+dan0js |access-date=2018-04-20 |archive-url=https://web.archive.org/web/20170922090210/http://dro.dur.ac.uk/5092/1/5092.pdf?DDD5+dan0js |archive-date=2017-09-22 |url-status=live }}</ref> Red can also affect the perception of dominance by others, leading to significant differences in mortality, [[reproductive success]] and [[parental investment]] between individuals displaying red and those not.<ref>{{Cite journal|last1=Cuthill|first1=I. C.|last2=Hunt|first2=S.|last3=Cleary|first3=C.|last4=Clark|first4=C.|year=1997|title=Colour bands, dominance, and body mass regulation in male zebra finches (Taeniopygia guttata)|journal=Proceedings of the Royal Society B: Biological Sciences|volume=264|issue=1384|pages=1093–99|doi=10.1098/rspb.1997.0151|pmc=1688540|bibcode=1997RSPSB.264.1093C}}</ref> In humans, wearing red has been linked with increased performance in competitions, including professional sport<ref>{{Cite journal |last1=Hill |first1=R. A. |last2=Barton |first2=R. A. |title=Psychology: Red enhances human performance in contests |journal=Nature |volume=435 |page=293 |year=2005 |doi=10.1038/435293a |bibcode=2005Natur.435..293H |issue=7040 |pmid=15902246|s2cid=4394988 |doi-access=free }}</ref><ref>{{cite journal |last1=Attrill |first1=M. |last2=Gresty |first2=K. |last3=Hill |first3=R. |last4=Barton |first4=R. |title=Red shirt colour is associated with long-term team success in English football |journal=Journal of Sports Sciences |year=2008 |volume=26 |issue=6 |pages=577–82 |doi=10.1080/02640410701736244 |pmid=18344128|s2cid=24581981 }}</ref> and [[multiplayer video game]]s.<ref>{{Cite journal |last1=Ilie |first1=A. |last2=Ioan |first2=S. |last3=Zagrean |first3=L. |last4=Moldovan |first4=M. |title=Better to Be Red than Blue in Virtual Competition |journal=CyberPsychology & Behavior |volume=11 |issue=3 |pages=375–77 |year=2008 |doi=10.1089/cpb.2007.0122 |pmid=18537513}}</ref> Controlled tests have demonstrated that wearing red does not increase performance or levels of testosterone during exercise, so the effect is likely to be produced by perceived rather than actual performance.<ref>{{Cite journal |last1=Hackney |first1=A. C. |title=Testosterone and human performance: influence of the color red |journal=European Journal of Applied Physiology |volume=96 |issue=3 |pages=330–33 |year=2005 |pmid=16283371 |doi=10.1007/s00421-005-0059-7|s2cid=22517777 }}</ref> Judges of [[tae kwon do]] have been shown to favor competitors wearing red protective gear over blue,<ref>{{Cite journal |last1=Hagemann |first1=N. |last2=Strauss |first2=B. |last3=Leissing |first3=J. |title=When the Referee Sees Red … |journal=Psychological Science |volume=19 |issue=8 |pages=769–71 |year=2008 |pmid=18816283 |doi=10.1111/j.1467-9280.2008.02155.x|s2cid=10618757 }}</ref> and, when asked, a significant majority of people say that red abstract shapes are more "dominant", "aggressive", and "likely to win a physical competition" than blue shapes.<ref name="Little & Hill 2007" /> In contrast to its positive effect in physical competition and dominance behavior, exposure to red decreases performance in cognitive tasks<ref>{{Cite journal |last1=Elliot |first1=A. J. |last2=Maier |first2=M. A. |title=Color and Psychological Functioning |journal=Current Directions in Psychological Science |volume=16 |issue=5 |pages=250–54 |year=2007 |doi=10.1111/j.1467-8721.2007.00514.x|s2cid=4678074 }}</ref> and elicits aversion in psychological tests where subjects are placed in an "achievement" context (e.g. taking an [[IQ test]]).<ref>{{Cite journal |last1=Elliot |first1=A. J. |last2=Maier |first2=M. A. |last3=Binser |first3=M. J. |last4=Friedman |first4=R. |last5=Pekrun |first5=R. |title=The Effect of Red on Avoidance Behavior in Achievement Contexts |journal=Personality and Social Psychology Bulletin |volume=35 |issue=3 |pages=365–75 |year=2008 |pmid=19223458 |doi=10.1177/0146167208328330|s2cid=14453487 }}</ref>