Shouldn't CMYK color be "additive"?

Answered by Stephen Beals, Digital Pre-Press manager and industry writer

Understanding additive and subtractive color is a bit counter intuitive. After all, aren't you adding ink to a press sheet, so shouldn't CMYK color be "additive"?

Well, no, not really.

Think of it as a matter of how much light is entering your eye (the way we humans perceive color).

If the sun is shining off water, what do you see? Mostly pure white. If the lights go off in the house at midnight, what do you see? Mostly black.

So the more light there is, the less color you see. The less light, the less color.

The most common mistake is to think we are talking about adding or subtracting COLOR. Color is merely the part of the light spectrum we perceive. What we are adding or subtracting is LIGHT.

A computer monitor is called additive color because it projects light directly at the eye. The more light it projects along the red, green, and blue color spectrum, the less "color" you actually see. Think of a camera exposure. An underexposed photo is too dark: not enough light is seen by the film or CCD. An overexposed photo ("adding light") is washed out. Not enough light = too dark. Too much light = too light.

I found another good way to understand additive color. You have probably been to a stage show in a theatre or sports arena where they use lights with colored gels. When they shine a red, green and blue light on the performer, you will notice that where one overlaps another, you get mixtures of color. A green light + a blue light = a yellow color. A red + blue yields a magenta hue, while green + red combines to create a yellow tint. Where all three colored spotlights shine as brightly as possible you get a white light shining on the performer.

In the printing process, we are dealing with the amount of light reflected by whatever light is in the environment (sunshine, fluorescent light, incandescent light, whatever). That light source bounces off the white paper (if indeed any paper can be called "white"). When we put ink on the paper, it effectively limits the amount of light that bounces off the sheet. That is, it "subtracts" light. So our color perception is the result of a "subtractive" process.

When you understand what the terms really mean, it is clear that black (K) is part of the subtractive print process. In theory you could get all of the colors needed from Cyan, Yellow, and Magenta. But inks have a lot of limitations. They must be transparent to allow the light to go through three pigments on the paper. Because of this, printing inks simply can't provide enough density to make colors "black" enough to look right to us. Therefore, black ink is added to the process.

But remember: adding black ink means SUBTRACTING the amount of light that bounces off the sheet.

Hope that helps clarify what is meant by additive and subtractive color.

Stephen Beals is a digital pre-press manager and has been writing for major print publications for many years. He is the author of A Practical Primer for Painless Print Production. He can be reached at
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