Not to be confused with the primary colors, which refer to light.
The primary pigments are also called the primary reflective colors, since they only have relevance in the context of light being reflected off of something to form visible color. A pigment is primary if no combination of other colors can be mixed to form it. It is for this reason that pigments' color is said to be subtractive. As you mix pigments they subtract in color until equal proportions of red, yellow, and blue give you black.
This is the basis of the universal four-color printing process CMYK (standing for cyan (blue), magenta (red), yellow, and black ("K", so as not to be confused with blue). a silk screen is used to create a plate of color, four of which (representing CMY and K) make up a photograph or print. A large range of colors can be represented using this method. You will note that black is used even though it could be produced by mixing equal amounts of CM and Y. This is because the mixture produces a muddy black, and purer black pigmentation exists.
In practice, mixing actual medium such as paint tends 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 progress like when mixing black and yellow in acrylic creates green.
For a more thorough and extensive research of color, see color.
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