SUBTRACTIVE COLOR SYNTHESIS uses paints, dyes, inks, and natural colorants to create color by absorbing some wavelengths of light and reflecting or transmitting others. This subtractive action is the basis of photographic filters, almost all films and color papers, and photomechanical reproduction in color.
White light is composed of all visible wavelengths, which can be divided into three primary-color bands, red, green and blue. A colorant that absorbs one wavelength band has the combined color of the other two; it is the complement of the color it subtracts from white light. Thus:
The complementary colors are the control colors of subtractive color synthesis; thus, the dyes in color filters and emulsions, and the inks (process colors) used in photomechanical reproduction are cyan, magenta, and yellow. A single complementary produces its own color. Two complementaries in equal strengths produce a primary color because each absorbs a primary--e.g., magenta and yellow absorb green and blue, respectively, leaving red to be seen. Combinations of unequal subtractive strengths produce intermediate colors from white light.
A combination of all three complementaries produces black (full strengths) or gray (lesser equal strengths) because all colors are subtracted. In color filtration this produces neutral density.
Primary-color lights can be additively mixed to produce colors, but primary-color dyes, inks, or filters do not permit selective color control by subtractive action because each absorbs the other two primaries equally. The complementary colors permit subtractive control of each of the three primaries individually; like additive synthesis, this corresponds with the three-color theory of vision.(1)
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(1) ICP Encyclopedia of Photography