Primary color
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The primary colors (or primary colours) are colors that cannot be created by mixing other colors in a given color space. 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 (i.e. subtractive primaries) 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.
[change] Additive primaries
Media 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 — a 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.
[change] 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.
Mixing 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. The abbreviation stands for "Cyan, Magenta, Yellow, and Black" — K stands for "Kohle" (German for coal) and 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 (in theory) change its hue but does reduce its saturation. In fact, adding white usually does change the hue slightly (this is known as the Abney effect), for example, adding white to red makes it slightly bluish, or pink.
