Primary color

A photograph of the red, green, and blue elements (subpixels) of an LCD. Additive mixing explains how light from these colored elements can be used for photorealistic color image reproduction.

The perception elicited by multiple light sources co-stimulating the same area of the retina is additive, i.e., predicted via summing the spectral power distributions (the intensity of each wavelength) or tristimulus values of the individual light sources (assuming a color matching context). For example, a purple spotlight on a dark background could be matched with coincident blue and red spotlights that are both dimmer than the purple spotlight. If the intensity of the purple spotlight was doubled it could be matched by doubling the intensities of both the red and blue spotlights that matched the original purple. The principles of additive color mixing are embodied in Grassmann's laws.[10]

Additive mixing of coincident spot lights was applied in the experiments used to derive the CIE 1931 colorspace. The original monochromatic primaries of the 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.[11]

Red, green, and blue light are popular primaries for additive color mixing since primary lights with those hues provide a large triangular chromaticity gamut.[12] Small red, green, and blue elements in electronic displays mix additively from an appropriate viewing distance to synthesize compelling colored images.[13]

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.

It is important to note that additive mixing provides very poor predictions of color perception outside the color matching context. Well known demonstrations such as The dress and other examples[14] show how the additive mixing model alone is not sufficient for predicting perceived color in many instances of real images. In general, we cannot completely predict all possible perceived colors from combinations of primary lights in the context of real-world images and viewing conditions. The cited examples suggest just how remarkably poor such predictions can be.

A magnified representation of small partially overlapping spots of cyan, magenta, yellow, and key (black) halftones in CMYK process printing. Each row represents the pattern of partially overlapping ink "rosettes" so that the patterns would be perceived as blue, green, and red when viewed on white paper from a typical viewing distance. The overlapping ink layers mix subtractively while additive mixing predicts the color appearance from the light reflected from the rosettes and white paper in between them.

The subtractive color mixing model predicts the resultant spectral power distribution of light filtered through 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, resulting in a colored appearance. The resultant spectral power distribution is predicted by sequentially taking the product of the spectral power distributions of the incoming light and transmissivity at each filter.[15] 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 may result in better color reproduction.

Cyan, magenta, and yellow are good subtractive primaries in that idealized filters with those hues can be overlaid to yield the largest chromaticity gamuts of reflected light.[16] An additional key ink (shorthand for the key printing plate that impressed the artistic detail of an image, usually black[17]) 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.[18]

A self-portrait by Anders Zorn clearly showing a four-pigment palette of what are thought to be white, yellow ochre, vermillion, and black pigments.[19][20]

Since the 18th century at least, the use of red, yellow, and blue as chromatic primaries has been regarded by many painters as sufficient for paintings.[21]

There are hundreds of commercially available pigments for visual artists to use and mix (in various media such as oil, watercolor, acrylic, gouache, and pastel). There is no specific set of pigments that are primary colors – the choice of pigments depends entirely on the artist's subjective preference of subject and style of art as well as material considerations like lightfastness and mixing heuristics. It is well known that a limited palette of white, red, yellow, and black and/or blue pigment is sufficient for many paintings.[22]

The color of light (i.e., the spectral power distribution) reflected from illuminated surfaces coated in paint mixes, slurries of pigment particles, is not well approximated by a subtractive or additive mixing model. Color predictions that incorporate light scattering effects of pigment particles and paint layer thickness require approaches based on the Kubelka–Munk[23] 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 perceptible effects on the way light is reflected from the paint. Artists typically rely on mixing experience and "recipes"[24] to mix desired colors from a small initial set of primaries and do not use mathematical modelling.

A contemporary description of the color vision system provides an understanding of primary colors that is consistent with modern color science. The human eye normally contains only three types of color photoreceptors, known as long-wavelength (L), medium-wavelength (M), and short-wavelength (S) cone cells. The response of these photoreceptor types varies across wavelengths of the visible electromagnetic spectrum. The S cone response is generally assumed to be negligible at long wavelengths greater than about 560 nm, while the L and M cones respond across the entire visible spectrum.[25] The LMS primaries are imaginary since there is no visible wavelength that stimulates only one type of cone (i.e., humans cannot normally see a color that corresponds to pure L, M or S stimulation). The LMS primaries are complete since every visible color can be mapped to a triplet specifying the coordinates in LMS color space.

Normalized cone spectral sensitivity curves

The L, M and S response curves (cone fundamentals) were deduced from color matching functions obtained from controlled color matching experiments (e.g., CIE 1931) where observers matched the color of a surface illuminated by monochromatic light with mixtures of three monochromatic primary lights illuminating a juxtaposed surface. Practical applications generally use a canonical linear 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. Any color space primaries which can be mapped to physiologically relevant LMS primaries by a linear transformation are necessarily either imaginary or incomplete or both. 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.

Humans are trichromats and use three (or more) primaries for many applications.[26] Two primaries would be unable to produce even some of the most common among the named colors. Adding a reasonable choice of the 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. There is one scholarly report of a functional human tetrachromat.[27]

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 non-human animals is challenging 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.[28]

Approximations within the sRGB gamut to the "aim colors" of the Natural Color System, a model based on the opponent process theory of color vision.

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 theory states that every color can be described as a mix along the three axes of red vs. green, blue vs. yellow and white vs. black. The six colors from the pairs might be called "psychological primary colors" because any other color could be described in terms of some combination of these pairs. Although there is a great deal of evidence for opponency in the form of neural mechanisms,[29] there is currently no clear mapping of the psychological primaries to neural substrates.[30]

The three axes of the psychological primaries were applied by Richard S. Hunter as the primaries for the colorspace ultimately known as CIELAB. The Natural Color System is also directly inspired by the psychological primaries.[31]

Historical accounts of primary colors can broadly be divided amongst those from philosophy, those related to the nature of light and color vision, and those that focus on pigments.

Philosophical writing from ancient Greece has described notions of primary colors but they can be difficult to interpret in terms of modern color science. Theophrastus (ca. 371–287 BCE) described Democritus’ position that the primary colors were white, black, red, and green.[32] In Classical Greece, Empedocles identified white, black, red, and, (depending on the interpretation) either yellow or green as primary colors.[33] Aristotle described an idea in which he thought white and black could be mixed in different ratios to yield chromatic colors; this idea had considerable influence in Western thinking about color.

Isaac Newton used the term "primary color" to describe the colored spectral components of sunlight.[34][35] A number of color theorists did not agree with Newton's work, David Brewster advocated that red, yellow, and blue light could be combined into any spectral hue late into the 1840s.[36][37] Thomas Young proposed red, green, and violet as the three primary colors, while James Clerk Maxwell favored changing violet to blue.[38] Hermann von Helmholtz proposed "a slightly purplish red, a vegetation-green, slightly yellowish, and an ultramarine-blue" as a trio.[39] Newton, Young, Maxwell, and Helmholtz were all prominent contributors to "modern color science"[40] that ultimately described the perception of color in terms of the three types of retinal photoreceptors.

John Gage's "The Fortunes Of Apelles" provides a summary of the history of primary colors[41] as pigments in painting and describes the evolution of the idea as complex. Gage begins by describing Pliny The Elder's account of notable Greek painters who used four primaries.[42] Pliny distinguished the pigments (i.e., substances) from their apparent colors: white from Milos (ex albis), red from Sinope (ex rubris), Attic yellow (sil) and atramentum (ex nigris). Sil was historically confused as a blue pigment between the 16th and 17th centuries leading to claims about white, black, red, and blue being the fewest colors required for painting. Thomas Bardwell, an 18th century Norwich portrait painter, was skeptical of practical relevance of Pliny's account.[43]

D'Aguilon's notion of the five primary colors (white, yellow, red, blue, black) was influenced by Aristotle's idea of the chromatic colors being made of black and white.[44] Robert Boyle, the Irish chemist, introduced the term primary color in English in 1664 and claimed that there were five primary colors (white, black, red, yellow, and blue).[45][46] The German painter Joachim von Sandart eventually proposed removing white and black from the primaries and that one only needed red, yellow, blue, and green to paint "the whole creation".[47]

The color scheme of François d'Aguilon, where the two simple colors of white (albus) and black (niger) are mixed to the "noble" colors of yellow (flavus), red (rubeus), and blue (caeruleus). Orange (aureus), purple (purpureus), and green (viridis) are each combinations of two noble colors.[48]

RYB color wheel

Red, yellow, and blue as primaries became orthodoxy amongst color theorists during the 18th and 19th centuries. Jacob Christoph Le Blon, an engraver, was the first to use separate plates for each color in mezzotint painting: yellow, red, and blue, plus black to add shades and contrast. Le Blon used primitive in 1725 to describe red, yellow, and blue in a very similar sense as Boyle used primary.[50] Moses Harris, an entomologist and engraver, also describes red, yellow, and blue as "primitive" colors in 1766.[51] Mérimée described red, yellow, and blue in his 1830 book on painting as the three simple/primitive colors that can make a "great variety" of tones and colors found in nature.[52] George Field, a chemist, used the word primary to describe red, yellow, and blue in 1835.[53] Michel Eugène Chevreul, also a chemist, also discussed red, yellow, and blue as "primary" colors in 1839.[54] [55]

Historical perspectives[56][57][58] on the earliest color order systems that were proposed in the 18th and 19th centuries describe them as using red, yellow and blue pigments as chromatic primaries. Tobias Mayer (a German mathematician, physicist, and astronomer) described a triangular bipyramid with red, yellow and blue at the 3 vertices in the same plane, white at the top vertex and black and the bottom vertex in a public lecture in 1758. There would be 11 planes of colors between the white and black vertices. Mayer did not seem to distinguish between colored light and colorant though he used vermilion, orpiment (King’s yellow), and Bergblau (azurite) in partially complete colorings of planes in his solid. Johann Heinrich Lambert (a Swiss mathematician, physicist, and astronomer) proposed a triangular pyramid with gamboge, carmine, and Prussian blue as primaries and only white at the top vertex (since Lambert could produce a mixture that was sufficiently black with those pigments). Lambert's work on this system was published in 1772. Philipp Otto Runge (the Romantic German painter) firmly believed in the theory of red, yellow and blue as the primary colors (again without distinguishing light color and colorant). His color sphere was ultimately described in an essay titled Farben-Kugel (color ball) published by Goethe in the book Farbenlehre in 1810. His spherical model of colors equally spaced red, yellow and blue longitudinally with orange, green and violet in between them and white and black at opposite poles. Runge's premature death prevented him from further investigation but his ideas are widely recognized as key insights that predated modern perceptual color models like the Munsell color system.

Albert Munsell, an American painter and creator of the aforementioned system , referred to the notion of red, yellow, and blue as primary colors as "mischief", "a widely accepted error", and underspecified[59] in his book A Color Notation first published in 1905.

Through the twentieth century and into the twenty-first, the RYB color model persists in traditional color theory (as opposed to modern color science), defining a designer's or painter's color wheel or color triangle; the secondary colors OGV (orange, green, and violet) make up another triad. Triads are formed by three equidistant colors on a particular color wheel; neither RYB nor OGV is equidistant on a perceptually uniform color wheel, but rather have been defined to be equidistant in the RYB wheel.[60]

Using red, yellow, and blue pigments or inks as primaries yields a relatively small gamut, in which, among other problems, colorful greens, cyans, purples and magentas may be impossible to mix. For this reason, modern three- or four-color printing processes, as well as color photography, use cyan, yellow, and magenta as primaries instead:[61] The cyan, magenta, and yellow used in printing are sometimes known as "process blue," "process red", and "process yellow."[62]

• Color vision
• RGB color model