<P> In the 18th century, the RYB primary colors became the foundation of theories of color vision, as the fundamental sensory qualities that are blended in the perception of all physical colors and equally in the physical mixture of pigments or dyes . These theories were enhanced by 18th - century investigations of a variety of purely psychological color effects, in particular the contrast between "complementary" or opposing hues that are produced by color afterimages and in the contrasting shadows in colored light . These ideas and many personal color observations were summarized in two founding documents in color theory: the Theory of Colors (1810) by the German poet and government minister Johann Wolfgang von Goethe, and The Law of Simultaneous Color Contrast (1839) by the French industrial chemist Michel - Eugène Chevreul . </P> <P> Painters have long used more than three RYB 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 . </P> <P> The cyan, magenta, and yellow primary colors associated with CMYK printing are sometimes known as "process blue", "process red" and "process yellow". </P>

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