Primary color

Primary colors are sets of colors that can be combined to make a useful range (gamut) of colors. For human applications, three are often used; for additive combination of colors, as in overlapping projected lights or in CRT displays, the primary colors normally used are red, green , and blue. For subtractive combination of colors, as in mixing of pigments or dyes, such as in printing, the primaries normally used are magenta, cyan, and yellow. [cite book | title = Color and Its Applications | author = Matthew Luckiesh | year = 1915 | publisher = D. Van Nostrand company | pages = pp. 58, 221 | url = http://books.google.com/books?id=0BgCAAAAYAAJ&pg=RA1-PA221&dq=magenta+cyan+yellow+date:0-1923+printing&as_brr=1 ]

Any choice of primary colors is essentially arbitrary; for example, an early color photographic process, autochrome, typically used orange, green, and violet primaries. [cite book | title = The World's Work: Volume XV: A History of Our Time | author = Walter Hines Page and Arthur Wilson Page | publisher = Doubleday, Page & Company | year = 1908 | url = http://books.google.com/books?id=hKPvxXgBN1oC&pg=PA9508&dq=autochrome+orange+violet+green&as_brr=1 ]

Biological basis

Primary colors are not a fundamental property of light but are often related to the physiological response of the eye to light. Fundamentally, light is a continuous spectrum of the wavelengths that can be detected by the human eye, an infinite-dimensional stimulus space.cite book | title = Light | author = Michael I. Sobel | publisher = University of Chicago Press | year = 1989 | isbn = 0226767515 | pages = 52–62 | url = http://books.google.com/books?id=PDmAdQpmxl8C&pg=PA58&ots=nx4W7J2aTc&dq=spectrum+color+infinite-dimensional+cones&sig=uM9RwCK7fFquO9e2oz-79xjbe8w#PPA59,M1 ] However, the human eye normally contains only three types of color receptors called cone cells. Each color receptor responds to different ranges of the color spectrum. Humans and other species with three such types of color receptors are known as trichromats. These species respond to the light stimulus via a three-dimensional sensation, which can generally be modeled as a mixture of three primary colors.

Species with different numbers of receptor cell types would have color vision requiring a different number of primaries. For example, for species known as tetrachromats, with four different color receptors, one would use four primary colors. Since humans can only see to 400 nanometers (violet), but tetrachromats can see into the ultraviolet to about 300 nanometers, this fourth primary color might be located in the shorter-wavelength range.

Many birds and marsupials are tetrachromats, and it has been suggested that some human females are tetrachromats as well [Backhaus, Kliegl & Werner "Color vision, perspectives from different disciplines" (De Gruyter, 1998), pp.115-116, section 5.5.] [Pr. Mollon (Cambridge university), Pr. Jordan (Newcastle university) "Study of women heterozygote for colour difficiency" (Vision Research, 1993)] , having an extra variant version of the long-wave (L) cone type. [cite journal | title = Expression of L cone pigment gene subtypes in females | journal = Vision Research | author = M. Neitz, T. W. Kraft, and J. Neitz | year = 1998 | volume = 38 | pages = 3221–3225 | doi = 10.1016/S0042-6989(98)00076-5 ] The peak response of human color receptors varies, even amongst individuals with “normal” color vision [Neitz, Jay & Jacobs, Gerald H. (1986). [http://www.nature.com/nature/journal/v323/n6089/abs/323623a0.html "Polymorphism of the long-wavelength cone in normal human colour vision."] "Nature". 323, 623-625.] ; in non-human species this polymorphic variation is even greater, and it may well be adaptive [Jacobs, Gerald H. (1996). [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=40094 "Primate photopigments and primate color vision."] "PNAS". 93 (2), 577–581.] .Most mammals other than primates have only two types of color receptors and are therefore dichromats; to them, there are only two primary colors.

It would be incorrect to assume that the world 'looks tinted' to an animal (or human) with anything other than the human standard of three color receptors. To an animal (or human) born that way, the world would look normal to it, but the animal's ability to detect and discriminate colors would be different from that of a human with normal color vision. If a human and an animal both look at a natural color, they see it as natural; however, if both look at a color reproduced via primary colors, for example on a color television screen, the human may see it as matching the natural color, while the animal does not; in this sense, reproduction of color via primaries must be "tuned" to the color vision system of the observer.

Additive primaries

Media that combine emitted lights to create the sensation of a range of colors are using the additive color system. Typically, the primary colors used are red, green, and blue.

Television and other computer and video displays are a common example of the use of additive primaries and the RGB color model. The exact colors chosen for the primaries are a technological compromise between the available phosphors (including considerations such as cost and power usage) and the need for large color triangle to allow a large gamut of colors. The ITU-R BT.709-5/sRGB primaries are typical.

Additive mixing of red and green light produces shades of yellow, orange, or brown. ["Some Experiments on Color", "Nature" 111, 1871, in cite book | title = Scientific Papers | author = John William Strutt (Lord Rayleigh) | year = 1899 | publisher = University Press | url = http://books.google.com/books?id=KWMSAAAAIAAJ&pg=PA84&dq=date:0-1923+light+red+green+yellow-or-orange&as_brr=1#PPA85,M1 ] Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple, including magenta. Mixing nominally equal proportions of the additive primaries results in shades of grey or white; the color space that is generated is called an RGB color space.

The CIE 1931 color space defines "monochromatic" primary colors with wavelengths of 435.8 nm (violet), 546.1 nm (green) and 700 nm (red). The corners of the color triangle are therefore on the spectral locus, and the triangle is about as big as it can be. No real display device uses such primaries, as the extreme wavelengths used for violet and red result in a very low luminous efficiency.

Subtractive primaries

Media that use reflected light and colorants to produce colors are using the subtractive color method of color mixing.

Traditional

RYB (red, yellow, and blue) is a historical set of subtractive primary colors. It is primarily used in art and art education, particularly painting. [cite book | title = Designer’s Color Manual: The Complete Guide to Color Theory and Application | author = Tom Fraser and Adam Banks | publisher = Chronicle Books | year = 2004 | isbn = 081184210X | url = http://books.google.com/books?id=WXZNPaX-LvcC&pg=PA27&ots=HShXs43Vb9&dq=red-yellow-blue+color+mixing&ei=Q5C7RpKQBaPKowLOzbnwBQ&sig=tzY-Dg0Vd2qsvzkAED_4kTV_AYE ] It predates modern scientific color theory.

RYB make up the primary color triad in a standard color wheel; the secondary colors VOG (violet, orange, and green) make up another triad. Triads are formed by 3 equidistant colors on a particular color wheel; neither RYB nor VOG are equidistant on a perceptually uniform color wheel, but rather have been "defined" to be equidistant in the RYB wheel. [cite book | title = Color Choices | author = Stephen Quiller | publisher = Watson–Guptill | year = 2002 | isbn = 0823006972 | url = http://books.google.com/books?id=jiUTZQj_v5QC&pg=PA12&ots=uIkYShJkkF&dq=what-is-a-color-wheel+spaced+red+yellow+blue&ei=PfO7RtDOOaDeoALSidXvBQ&sig=nKVzb_VaCzhkW5LkewElB4laG90 ]

Painters have long used more than three “primary” colors in their palettes—and at one point considered red, yellow, blue, and green to be the "four" primaries [For instance Leonardo da Vinci wrote of these four simple colors in his notebook circa 1500. See Rolf Kuenhi. “Development of the Idea of Simple Colors in the 16th and Early 17th Centuries”. "Color Research and Application". Volume 32, Number 2, April 2007.] . Red, yellow, blue, and green are still widely considered the four psychological primary colors [cite book | title = Basic Photographic Materials and Processes | author = Resultby Leslie D. Stroebel, Ira B. Current| publisher = Focal Press| year = 2000 | isbn = 0240803450 | url = http://books.google.com/books?id=BRYa6Qpsw48C&pg=PP1&dq=Basic+Photographic+Materials+and+Processes&sig=3FfkDIRvz8MSinhegznHIKn4AvM] , though red, yellow, and blue are sometimes listed as the "three" psychological primaries [cite book | title = Decorative Painting & Faux Finishes | author = MS Sharon Ross, Elise Kinkead| publisher = Creative Homeowner| year = 2004 | isbn = 1580111793 | url = http://books.google.com/books?id=DPJUWRydR9kC&dq=red+yellow+blue+paint-mixing++subtractive&as_brr=3] , with black and white occasionally added as a fourth and fifth [cite book | title = Dimensional Color | author = Swirnoff, Lois| publisher = W. W. Norton & Company| year = 2003 | isbn = 0393731022 | url = http://books.google.com/books?id=sG5MqtZuFF0C&dq=%22psychological+primaries%22+blue+-green] .

During the 18th century, as theorists became aware of Isaac Newton’s scientific experiments with light and prisms, red, yellow, and blue became the canonical primary colors—supposedly the fundamental sensory qualities that are blended in the perception of all physical colors and equally in the physical mixture of pigments or dyes. This theory became dogma, despite abundant evidence that red, yellow, and blue primaries cannot mix all other colors, and has survived in color theory to the present day. [Bruce MacEvoy. “Do ‘Primary’ Colors Exist?” ( [http://www.handprint.com/HP/WCL/color6.html#materialtrichromacy Material Trichromacy section] ). "Handprint". Accessed 10 August 2007.]

Using red, yellow, and blue as primaries yields a relatively small gamut, in which, among other problems, colorful greens, cyans, and magentas are impossible to mix, because red, yellow, and blue are not well-spaced around a perceptually uniform color wheel. For this reason, modern three- or four-color printing processes, as well as color photography, use cyan, yellow, and magenta as primaries instead. [“Development of the Idea of Simple Colors in the 16th and Early 17th Centuries”. "Color Research and Application". Volume 32, Number 2, April 2007.] Most painters include colors in their palettes which cannot be mixed from yellow, red, and blue paints, and thus do not fit within the RYB color model. Some who do use a three-color palette opt for the more evenly spaced cyan, yellow, and magenta used by printers, and others paint with 6 or more colors to widen their gamuts. [Bruce MacEvoy. “ [http://www.handprint.com/HP/WCL/palette4e.html Secondary Palette] .” "Handprint". Accessed 14 August 2007. For general discussion see Bruce MacEvoy. “Mixing With a Color Wheel” ( [http://www.handprint.com/HP/WCL/color14.html#satcost Saturation Costs section] ). "Handprint". Accessed 14 August 2007.] The cyan, magenta, and yellow used in printing are sometimes known as "process blue," "process red," "process yellow." [ [http://www.printoutlet.us/glossary.php?glossaryid=2526 Cheap Brochure Printing - Process Blue / Process Red / Process Yellow / Process Black ] ]

CMYK color model, or four-color printing

In the printing industry, to produce the varying colors the subtractive primaries cyan, magenta, and yellow are applied together in varying amounts. Before the color names "cyan" and "magenta" were in common use, these primaries were often known as blue-green and purple, or in some circles as blue and red, respectively, and their exact color has changed over time with access to new pigments and technologies. [cite book | title = General Physics and Its Application to Industry and Everyday Life | author = Ervin Sidney Ferry | publisher = John Wiley & Sons | year = 1921 | url = http://books.google.com/books?id=3rYXAAAAIAAJ&pg=PA621&dq=date:0-1923+additive+color+mixing+primary&as_brr=1 ]

Mixing yellow and cyan produces green colors; mixing yellow with magenta produces reds, and mixing magenta with cyan produces blues. In theory, mixing equal amounts of all three pigments should produce grey, resulting in black when all three are applied in sufficient density, but in practice they tend to produce muddy brown colors. For this reason, and to save ink and decrease drying times, a fourth pigment, black, is often used in addition to cyan, magenta, and yellow.

The resulting model is the so-called CMYK color model. The abbreviation stands for cyan, magenta, yellow, and key—black is referred to as the "key" color, a shorthand for the "key printing plate" that impressed the artistic detail of an image, usually in black ink. [cite book | title = Printing for School and Shop: A Textbook for Printers' Apprentices, Continuation Classes, and for General use in Schools | author = Frank S. Henry | publisher = John Wiley & Sons | year = 1917 | url = http://books.google.com/books?id=UAAvAAAAMAAJ&pg=PA292&dq=black+date:0-1923+key-plate+printing+color ]

In practice, colorant mixtures in actual materials such as paint tend to be more complex. Brighter or more saturated colors can be created using natural pigments instead of mixing, and natural properties of pigments can interfere with the mixing. For example, mixing magenta and green in acrylic creates a dark cyan—something which would not happen if the mixing process were perfectly subtractive.

In the subtractive model, adding white to a color, whether by using less colorant or by mixing in a reflective white pigment such as zinc oxide, does not change the color’s hue but does reduce its saturation. Subtractive color printing works best when the surface or paper is white, or close to it.

A system of subtractive color does not have a simple chromaticity gamut analogous to the RGB color triangle, but a gamut that must be described in three dimensions. There are many ways to visualize such models, using various 2D chromaticity spaces or in 3D color spaces. [See the [http://images.google.com/images?q=cmyk%20gamut google image results] for “cmyk gamut” for examples.]

Four "pure" colors

Psychovisual studies and the opponent process color model lead to the notion of four "pure" or "unique" colors: [cite book | title = Sensation and Perception | author = E. Bruce Goldstein | Publisher = Wadsworth Publishing Co | year = 1989 | edition = 3rd ed. | isbn = 0534096727] red, yellow, green, and blue, with red and green defining one color-opponent axis, and yellow and blue a second color-opponent axis.

See also

*Color theory
*Secondary color
*Tertiary color
*Visual perception

Notes and references

External links

* [http://handprint.com/HP/WCL/wcolor.html Handprint : do "primary" colors exist?] – a comprehensive site on color primaries, color perception, color psychology, color theory and color mixing.
* [http://www.newton.dep.anl.gov/askasci/phy00/phy00871.htm Ask A Scientist: Primary Colors]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/vision/colcon.html#c1 The Color-Sensitive Cones at HyperPhysics]
* [http://www.tomjewett.com/colors/index.html Color Tutorial]