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primary colors[′prī‚mer·ē ′kəl·ərz]
three colors that can be optically mixed in such a way that the color obtained is visually indistinguishable from any given color. A color C that is being measured or reproduced can be represented in terms of the primary colors E1, E2, and E3 as the sum C that is indistinguishable from C; C’ = m1E1 + m2E2 + m3E3, where m1, m2, and m3 are the quantities of the primary colors being mixed. These quantities are called the coordinates of C (one and sometimes two coordinates can be negative).
A set of primary colors constitutes a tristimulus colorimetric system. There is an infinite number of possible systems of primary colors. However, not just any set of three different, arbitrarily selected colors can serve as a set of primary colors, because no one of the three primary colors should be represented as the sum of any quantities of the other two colors. A color C can be defined in any tristimulus system; the transition from one system to another is achieved through simple relationships. To compare color measurements of different systems it is customary to recompute the results in units of a standard international system with the primary colors X, Y, and Z (these colors do not correspond to any visually perceived real colors).
The three primary colors most frequently used are red, green, and blue. The colors purple (blue-red), azure (green-blue), and yellow are used in multicolor printing and in color photography, where colors are reproduced by the more complicated process of substractive mixing.
The concept of primary colors and the relationships based on this concept are widely used in theoretical and practical colorimetry. However, the methods of tristimulus systems are inadequate for the solution of a number of subtle, complex problems associated with measurements and quantitative description of colors.