Primary Colors

Primary colors are sets of colors that can be combined to make a useful range 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 cyan, magenta, and yellow

Any choice of primary colors is essentially arbitrary; for example, an early color photographic process, autochrome, typically used orange, green, and violet primaries.
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. 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 generally can be modeled as a mixture of three primary colors.


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. 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
RYB (red, yellow, and blue) is a historical set of subtractive primary colors. It is primarily used in art and art education, particularly painting. 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 is equidistant on a perceptually uniform color wheel, but rather have been defined to be equidistant in the RYB wheel.

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. Red, yellow, blue, and green are still widely considered the four psychological primary colors, though red, yellow, and blue are sometimes listed as the three psychological primaries, with black and white occasionally added as a fourth and fifth.

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

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. 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 The cyan, magenta, and yellow used in printing are sometimes known as "process blue," "process red," "process yellow."