that the combination of any pair of the subtractive primary colors in equal densities produces one of the
additive primary colors.
(1) A yellow filter transmits red and green and absorbs blue, and a magenta filter transmits
red and blue and subtracts green from the light source. When these two filters are used over a single
light source, the one color that is transmitted by both magenta and yellow is red. Therefore, yellow plus
magenta produces red.
(2) In the same manner, when yellow and cyan are used in combination, the one color that is
transmitted by both filters is green; since yellow transmits red and green, and cyan transmits blue and
(3) Cyan plus magenta produces blue, because blue is transmitted by both filters. Where all
three filters overlap in the center, all of the light is absorbed, and the result is black.
(4) By varying the density of any one of the filters, any desired change in the color produced
can be brought about. For example, to change the appearance of red to make it an orange red, increase
the amount of yellow; in other words, decrease the amount of magenta.
The principal theory of filters is that a filter transmits its own color and absorbs all others (table
2-1). A red filter will transmit red and absorb blue and green and the color they have in common is cyan.
A cyan filter will transmit blue and green but will absorb red which is its compliment. When a CP 30C
filter (fig 2-4) is placed over a white light source in subtractive color printing, it will transmit almost all
the green and blue and subtract part of the red. In this case it will be 0.30 log exposure unit.
NOTE: "CP" stands for color printing, "30" for a density of 0.30, and "C" for cyan.
Figure 2-4. Color printing filter