A set of primary colors is a set of pigments, colored lights, or abstract elements of a mathematical color space model that can be combined in varying amounts to produce a range or "gamut" of colors. Deriving many colors from several primaries facilitates technological and artistic applications such as painting, electronic displays, and printing. Any small set of realizable primary colors are "imperfect" in that they cannot generate all perceptible colors, but some sets of primaries can yield a far wider gamut than others.
For an additive set of primary colors for human vision, as in a television or computer display screen, projector, or other emissive electronic visual displays, the usual choice is red, green, and blue (RGB), although the primaries' specific chromaticities can vary.
For a set of subtractive primary colors for humans, as in mixing of pigments for printing, cyan (a bright greenish-blue), magenta (a bright reddish purple), and yellow are often used (usually supplemented by black to make CMYK).
Red, yellow, and blue (RYB) are a well-known traditional set of subtractive primaries in the art field. Artists often use more than three chromatic pigments and so are not limited to a colorspace such as RYB even if they think of these three as their primary colors in an abstract or conceptual sense, for example when constructing a color wheel.
The precise set of primary colors to be used in a specific color application depends on gamut requirements as well as on application-specific constraints such as cost, power consumption, lightfastness, mixing behavior, etc.
Abstract colorspaces include the additive CIE XYZ, in which all visible colors can be represented mathematically by combinations of its X, Y, and Z primaries, but each primary itself does not directly correspond to a physically-possible light source.
Color applications based on primary mixing
The response of the eye to a "mix" of primary colors is predicted by different models for different applications.
Additive mixing of light
In additive color mixing, the total light present is simply the sum of individual light sources. For example, coincident blue and red spotlights on a surface will make a purple, brighter than either of the spotlights alone. Additive mixing of coincident spot lights was applied in the experiments used to derive the CIE 1931 colorspace. The original monochromatic primaries of the (arbitrary) wavelengths of 435.8 nm (violet), 546.1 nm (green) and 700 nm (red) were used in this application due to the convenience they afforded to the experimental work.
Red, green, and blue light are the ideal primaries for additive color mixing since primary lights with those hues provide the largest triangular chromaticity gamuts. Small red, green, and blue elements in electronic displays mix additively in the eye at an appropriate viewing distance to synthesize compelling colored images.
The exact colors chosen for additive primaries are a technological compromise between the available phosphors (including considerations such as cost and power usage) and the need for large chromaticity gamut. The ITU-R BT.709-5/sRGB primaries are typical.
Subtractive mixing of ink layers
Subtractive color mixing describes overlaid partially absorbing materials on a reflecting or transparent surface. Each layer partially absorbs some wavelengths of light from the illumination spectrum while letting others pass through (combining multiplicatively), which results in a colored appearance. Overlapping layers of ink in printing mix subtractively over reflecting white paper in this way, to generate photorealistic color images. The typical number of inks in such a printing process ranges from 3 to 6 (e.g., CMYK process, Pantone hexachrome). In general, using fewer inks as primaries results in more economical printing but using more achieves potentially better color reproduction.
Cyan, magenta, and yellow are good subtractive primaries in that the reflectance curves corresponding to idealized inks can be combined for the largest chromaticity gamuts. An additional key ink (shorthand for the key printing plate that impressed the artistic detail of an image, usually black,) is also usually used since it is difficult to mix a dark enough black ink using the other three inks. Before the color names cyan and magenta were in common use, these primaries were often known as blue and red, respectively, and their exact color has changed over time with access to new pigments and technologies.
Mixing paints in limited palettes
The perceived color of mixed paints, slurries of pigment particles typically suspended in water or linseed oil, isn't well approximated by either subtractive or additive mixing model. Color predictions that incorporate light scattering effects of pigment particles and paint layer thickness require approaches based on the Kubleka-Munk equations. Even such approaches cannot predict the color of paint mixtures precisely since small variances in particle size distribution, impurity concentrations etc. can be difficult to measure but impart significant effects on the way light is reflected from the paint. Artists typically rely on mixing experience and "recipes" to mix desired colors from a small initial set of primaries and do not use mathematical modelling.
There are hundreds of commercially available pigments for visual artists to use and mix (in various media such as oil, watercolor, acrylic, and pastel). A common approach is to use just a limited palette of pigments (often between four and eight) that can be physically mixed to any color that the artist desires in the final work. There are no specific set of pigments that are primary colors, the choice of pigment depends entirely on the artist's subjective preference of subject and style of art as well as material considerations like lightfastness and mixing heuristics. Contemporary classical realists have often advocated that a limited palette of white, red, yellow, and black pigment (often described as the "Zorn palette") is sufficient for compelling work.
A chromaticity diagram can illustrate the gamut of different choices of primaries, for example showing which colors are lost (and gained) if you use RGB for subtractive color mixing (instead of CMY).
A contemporary description of the color vision system provides an understanding of the primary colors that is consistent with modern color science. The human eye normally contains only three types of color photoreceptors (L, M, and S) that are associated with specialized cone cells. These photoreceptor types respond to different (though overlapping) ranges of the visible electromagnetic spectrum, and there is no single wavelength that stimulates only one photoreceptor type. Humans and other species with three such types of color photoreceptor are known as trichromats.
Although color perception is a complex biological process, controlled color matching experiments (e.g., CIE 1931) have essentially mapped all visible colors in that limited context. The results from those experiments can ultimately be mathematically transformed into the average spectral sensitivity curves (cone fundamentals) for each of the three color photoreceptors. L, M, and S are the "primaries" of LMS space since essentially any color can be specified by a weighted vector sum of the L, M, and S responses for that color. Practical applications generally use a canonical transformation of LMS space known as CIEXYZ. The X, Y, and Z primaries are typically more useful since luminance (Y) is specified separately from a color's chromaticity (X and Z). There are many useful derivatives of CIEXYZ including CIELUV and CIELAB, each colorspace possessing its own set of primaries that are used to specify essentially any color. All of respective primaries of these colorspaces which are conceptually rooted in LMS space are necessarily abstract in that weighted sums of the primaries can specify essentially any color but there are no physically realizable colors corresponding to the primaries themselves. All of these primaries are also arbitrary in the sense that they can be subjected to various types of mathematical transformations and still specify each color in the original space exactly and completely, i.e. no set of such primaries can specify more colors than another. The color matching context is always three dimensional (since LMS space is three dimensional) but more general color appearance models like CIECAM02 describe color in six dimensions and can be used to predict how colors appear under different viewing conditions.
For real (dye/pigment/light source) primary colors (as opposed to abstract like XYZ), the number of color receptors becomes the minimum number of primaries needed in order to produce more than a tiny sliver of the perceptible color gamut. Thus for trichromats like humans, we use three (or more) primaries for most general purposes. Two primaries would be unable to produce even some of the most common among the named colors. Adding a reasonable choice of third primary can drastically increase the available gamut, while adding a fourth or fifth may increase the gamut but typically not by as much.
Most placental mammals other than primates have only two types of color photoreceptor and are therefore dichromats, so it is possible that certain combinations of just two primaries might cover some significant gamut relative to the range of their color perception. Meanwhile, birds and marsupials have four color photoreceptors in their eyes, and hence are tetrachromats with a more complex colour perception system. There is one scholarly report of a functional human tetrachromat.
The presence of photoreceptor cell types in an organism's eyes do not directly imply that they are being used to functionally perceive color. Measuring functional spectral discrimination in any animal is difficult due to the difficulty in performing psychophysical experiments on creatures with limited behavioral repertoires who cannot respond using language. Limitations in the discriminative ability of shrimp having twelve distinct color photoreceptors have demonstrated that having more cell types in itself need not always correlate with better functional color vision.
There are numerous competing primary colour systems throughout history. Scholars and scientists engaged in debate over which hues best describe the primary color sensations of the eye. Thomas Young proposed red, green, and violet as the three primary colors, while James Clerk Maxwell favoured changing violet to blue. Hermann von Helmholtz proposed "a slightly purplish red, a vegetation-green, slightly yellowish, and an ultramarine-blue" as a trio. In modern understanding, human cone cells do not correspond precisely to a specific set of primary colors, as each cone type responds to a relatively broad range of wavelengths.
The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from cones and rods in an antagonistic manner. The three types of cones have some overlap in the wavelengths of light to which they respond, so it is more efficient for the visual system to record differences between the responses of cones, rather than each type of cone's individual response. The opponent color theory suggests that there are three opponent channels: red versus green, blue versus yellow, and black versus white. Responses to one color of an opponent channel are antagonistic to those of the other color. The theory states that the particular colors considered by an observer to be uniquely representative of the concepts red, yellow, green, blue, white, and black might be called "psychological primary colors", because any other color could be described in terms of some combination of these.
- Matthew Luckiesh (1915). Color and Its Applications. D. Van Nostrand company. pp. 58, 221.
- Thomas D. Rossing & Christopher J. Chiaverina (1999). Light science: physics and the visual arts. Birkhäuser. p. 178. ISBN 978-0-387-98827-6.
- Frank S. Henry (1917). Printing for School and Shop: A Textbook for Printers' Apprentices, Continuation Classes, and for General use in Schools. John Wiley & Sons.
- Ervin Sidney Ferry (1921). General Physics and Its Application to Industry and Everyday Life. John Wiley & Sons.
- Nyholm, Arvid (1914). "Anders Zorn: The Artist and the Man". Fine Arts Journal. 31 (4): 469. doi:10.2307/25587278.
- Kubelka, Paul; Munk, Franz (1931). "An article on optics of paint layers" (PDF). Z. Tech. Phys. 12: 593–601.
- Gurney. "The Zorn Palette". Gurney Journey. Retrieved 27 September 2016.
- Steven Westland, "subtractive mixing – why not RGB?", October 4, 2009 http://colourware.org/2009/10/04/subtractive-mixing-why-not-rgb/
- Bruce MacEvoy. "Do 'Primary' Colors Exist?" (Material Trichromacy section). Handprint. Accessed 10 August 2007.
- Jordan, G.; Deeb, S. S.; Bosten, J. M.; Mollon, J. D. (20 July 2010). "The dimensionality of color vision in carriers of anomalous trichromacy". Journal of Vision. 10 (8): 12–12. doi:10.1167/10.8.12.
- Morrison, Jessica (23 January 2014). "Mantis shrimp's super colour vision debunked". Nature. doi:10.1038/nature.2014.14578.
- Edward Albert Sharpey-Schäfer (1900). Text-book of physiology. 2. Y. J. Pentland. p. 1107.
- Alfred Daniell (1904). A text book of the principles of physics. Macmillan and Co. p. 575.
- Michael Foster (1891). A Text-book of physiology. Lea Bros. & Co. p. 921.