acc. to dictionary;
primary color
n.
A color belonging to any of three groups each of which is regarded as generating all colors, with the groups being:
1 Additive, physiological, or light primaries red, green, and blue. Lights of red, green, and blue wavelengths may be mixed to produce all colors.
2 Subtractive or colorant primaries magenta, yellow, and cyan. Substances that reflect light of one of these wavelengths and absorb other wavelengths may be mixed to produce all colors.
3 Psychological primaries red, yellow, green, and blue, plus the achromatic pair black and white. All colors may be subjectively conceived as mixtures of these.
A primary color (or colour) is a color that cannot be created by mixing other colors in the gamut of a given color space. Primary colors may themselves be mixed to produce most of the colors in a given color space: mixing two primary colors produces what is generally called a secondary color, mixing a secondary with a primary produces what is sometimes called a tertiary color. Traditionally, the colors red, yellow, and blue are considered to be primary pigments in the art world. However those colors are not the same hue as the "red", "yellow" and "blue" used in alternate color systems. Many modern applications use primary additive colors of red, green and blue; and the primary pigments of magenta, yellow, and cyan. If the color space is considered as a vector space, the primary colors can be regarded as a set of basis vectors for that space.
Biological basis
Primary colors are not a physical but rather a biological concept, based on the physiological response of the human eye to light. Light represents a continuous spectrum of wavelengths, such that being "primary" is not an intrinsic property of the color itself. The human eye contains receptors called cones which normally respond to specific wavelengths of red, green, and blue light. Humans and other species with three such types of color receptors are known as trichromats. Although the peak responsivities of the cones do not occur exactly at the red, green and blue frequencies, those three colors are chosen as primary because they provide a wide gamut, making it possible to almost independently stimulate the three color receptors. To generate optimal color ranges for species other than humans, other additive primary colors would have to be used. For species known as tetrachromats with four different color receptors, one would use four primary colors. Many birds and marsupials are tetrachromats and it has been suggested that some female humans are born as tetrachromats as well, having an extra receptor for yellow. On the other hand, most mammals have only two types of color receptors and are therefore dichromats; to them, there are only two primary colors.
Additive primaries
Additive color mixingMedia that combine emitted lights to create the sensation of a range of colors are using the additive color system. Television is the most common use of this. The Additive primaries are red, green, and blue. Because of the response curves of the three different color receptors in the human eye, these colors are optimal in the sense that the largest range of colors (gamut) visible by humans can be generated by mixing light of these colors. Additive mixing of red and green light, produce shades of yellow or orange. Mixing green and blue produces shades of cyan, and mixing red and blue produces shades of purple and magenta. Mixing equal proportions of the additive primaries results in shades of grey; when all three colors are fully saturated, the result is white. The color space that is generated is called the RGB ("red, green, blue") color space.
Subtractive primaries
Media that use reflected light and colorants to produce colors are using the subtractive color method of color mixing. In the printing industry, to produce the varying colors, apply the subtractive primaries yellow, cyan, and magenta together in varying amounts. Subtractive color works best when the surface (or paper) is white, or close to it.
Subtractive color mixingMixing yellow and cyan produces shades of green; mixing yellow with magenta produces shades of red, and mixing magenta with cyan produces shades of blue. In theory, mixing equal amounts of all three pigments should produce shades of grey, resulting in black when all three are fully saturated, but in practice they tend to produce muddy brown colors. For this reason, a fourth "primary" pigment, black, is often used in addition to the cyan, magenta, and yellow colors. The color space generated is the so-called CMYK color space. (standing for "Cyan, Magenta, Yellow, and Black - K is used to represent black as 'B' could be confused with 'Blue'").
In practice, mixtures of actual materials like paint tend to be less precise. Brighter, or more specific 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 does not change its hue but does reduce its saturation.
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