Although white light contains a rainbow of colors, our eyes are only sensitive to red (R), green (G), and blue (B) light. Your brain takes the stimuli from those three colors and interprets all the other colors based on the proportion of R to G to B. Since human brains only process those three colors of light, white light can effectively be described as a combination of RGB. (Click to enlarge diagram below.)
Red, green, and blue are known as the primary colors of light. The combinations of two of the three primary colors of light produce the secondary colors of light. The secondary colors of light are cyan, magenta, and yellow. In printing, the abbreviation for cyan is C, magenta is M, and yellow is Y.
Black is usually used in conjunction with cyan, magenta, and yellow to provide image details (click to enlarge diagram below). In printing, the abbreviation for black is K—K stands for key or key color. (K is also used to avoid confusion with blue.) You might see the initials of these colors, CMYK, in association with various printing procedures, processes, and products.
The secondary colors of light are the primary colors of pigments or dyes (not red, yellow, and blue, as many people are taught). If white light interacts with the primary colors of pigments, primary colors of light are removed.
Printed materials, like the acetates in this activity, use CMYK's ability to absorb or filter RGB colors to produce a multitude of other colors. This principle is called color subtraction (your printer uses CMYK pigments and color substitution to make the pictures you print).
What happens to white light as it passes through colored acetate?
The acetates you printed for this activity have regions coated with ink (cyan, magenta, yellow, or black pigments) and other regions that are uncoated or clear. The amount of pigment affects the color and intensity of the light that passes through it.
For example, if a cyan sheet is held up to the light, all colors of white light pass through the clear or uncoated areas. However, in the portions of the sheet where the cyan pigment is more intensely coated, virtually all red light will be blocked. If the magenta sheet is held up to the light, the green light will be blocked. The yellow sheet blocks the blue light. (Click to enlarge diagrams below.)
If two layers are aligned, the primary colors of light may appear. For example, if a solidly colored portion of cyan acetate is held against a solidly colored portion of yellow acetate, only green light is seen. This is because cyan subtracts red light and yellow subtracts blue light, allowing only green light to pass through. (Click to enlarge diagram below.)
When all CMYK acetates are overlaid, depending on where you look on the acetates, various colors and various intensities are subtracted from the white light. This allows you to make an infinite number of colors from different combinations of CMYK.