Two Laws of MixingAdditive and Subtractive

The previous chapter mixed colors only in the mind’s eye. Now the physical mixtures — and the awkward fact that there are two of them, pulling in opposite directions.

Mixing light

Project beams of colored light onto a screen so they overlap. Every overlap is lighter than either beam that produced it. Combine all the colors of the spectrum and you get white. The sum of all colored light is white. This is additive mixture: adding light yields more light.

This is the law that governs every screen you look at. A pixel that appears yellow is a red sub-pixel and a green sub-pixel emitting together. There is no yellow ink in your phone.

Mixing pigment

Now mix paint on a palette. The result is always darker than either ingredient. The more pigments you combine, the muddier and darker the mixture gets. Combine enough and you converge on black. This is subtractive mixture: each pigment absorbs some wavelengths, and the more you add, the more is absorbed.

Toggle the same two parent colors between the two laws and watch the result jump — lighter toward white under addition, darker toward black under subtraction.

Why two laws?

The split is not arbitrary. Light sources emit photons; the more sources, the more photons, the brighter the result. Pigments and filters absorb photons; the more material, the more absorption, the darker the result. The laws point in opposite directions because emission and absorption are opposite processes.

There is a third case worth knowing. Spin two patches of reflected color on a fast disc and the eye fuses them into one. This loses less light than stirring pigments together in a pot, so the result settles near a middle grey rather than sinking into dark mud. The rule generalizes: light mixtures gain light, pigment mixtures lose it, optical mixtures split the difference.

Knowing which law applies decides which way the brightness will move. Get the law wrong and every prediction about a color combination will be systematically off.