There are other aspects of light and color, and to understand them we must return to Sir Isaac's spectrum. Not all of the colors of the spectrum have "equal status".

There is a basic difference between the color behavior of light and the color behavior of the "red" apple. It is basically a difference between something that adds or subtracts. Color that results from adding light energy is called additive color. Color that results when an object subtracts light energy is called subtractive color.

A light source, such as the sun or a light bulb, is additive energy. Two separate light sources contribute more energy than one light source. Yellow is perceived when a green light source adds its energy to the energy of a red light source. The result is obtained by the addition of energy of differing light wavelengths.

 

It can be demonstrated that three colors, red, green and blue, when suitably combined, will reproduce white or colorless light. (Figure 5) Also by combining red, green, blue in different proportions any of the other colors of the spectrum can be produced.
Thus, red, green and blue are called the primary colors of the visible spectrum. A combination of any two primaries produces an intermediate color which we call a secondary color. Red and blue produce magenta, green and blue produce cyan, and, believe it or not, green and red produce yellow. (Figure 7)The red apple in Figure 3 subtracts energy. It has no light energy of its own, but modifies light contributed to it from a radiating source, such as the sun or a light bulb.

Inks, pigments, apples, and almost anything else you can think of are perceived as a certain color because we "see" the wavelengths they reflect or transmit. We do not "see" the wavelengths that are absorbed.

It is particularly important for us to also understand the subtractive method of obtaining color, for in the graphic arts we are concerned with such light absorbing materials as pigments, films and paper surfaces. All materials that are not sources of light energy operate by the subtractive method.

As there are three additive primary colors of light (red, green and blue), which add their energies to produce all the other colors of the visible spectrum, so three subtractive colors may be selected which, in proper combination, duplicate the colors of the visible spectrum. (Figures 1, 2 and Figure 7)

These subtractive secondary colors are commonly known as cyan, magenta and yellow. Cyan contains blue and green; magenta contains red and blue. Each process printing ink, cyan, magenta and yellow, ideally transmits two-thirds of the spectrum and absorbs one-third of the spectrum, that is cyan (blue/green) absorbs red; magenta (red/blue) absorbs green; and yellow (red/green) absorbs blue. (See Figure 7)

As in Figure 9 (a) cyan has the property of transmitting blue and green and absorbing red. Yellow has the property of transmitting red and green and absorbing blue. The only color that both have in common or that both can transmit is green- hence the visual result of overlapping these two subtractive secondaries is green. In (b) we have overlapped all three subtractive secondaries, there is no one color common to all three; therefore, they in effect cancel each other out and the result is black.

(c) and (d) illustrate that subtractive secondaries are needed to make the reproduction of other colors possible by the subtractive process. In (c) we have overlapped blue and red. Each is a single color representing and transmitting only one-third of the spectrum, absorbing two-thirds. Therefore, they cancel each other out, for they in effect have nothing in common. A third color results from the overlapping of two colors that have a color in common. See (d).

We have adopted a convenient device of dividing the spectrum into thirds. Each of the subtractive secondary colors of light composes, for practical purposes, one third of the visible spectrum. Each of the subtractive primary colors represents two-thirds of the spectrum, which they must do in order to make the subtractive process work efficiently. (Figure 6)