Subtractive colour

Ink does not emit anything. A layer of cyan on white paper absorbs most of the red arriving from the room and reflects the rest, so what reaches the eye is whatever the paper had minus what the ink took. Add magenta over it and the green goes too, and the result is a dark blue rather than a brighter anything: every layer can only subtract, so the stack runs towards black by construction. Cyan and yellow give green, magenta and yellow give red, cyan and magenta give blue, and the three together land on a muddy near-black. Not an actual black, which is the first practical fact about the model: real inks are not ideal absorbers, three of them at full strength make a heavy brown sludge that takes forever to dry, and that is exactly why print carries a fourth black ink and why a rich black is mixed rather than printed from the black plate alone.

The consequence designers meet is gamut loss, and it hurts in one direction in particular. The saturated colours a screen is best at are the ones ink is worst at: an emitted blue or a fluorescent green sits well outside anything three absorbing layers can reach, so it comes back from the press dull, dark, and slightly the wrong hue. The reverse problem barely exists, because almost every colour ink can make is inside a screen’s reach. So the honest workflow is to design towards the smaller space when the piece will be printed, proof it, and treat the screen preview as optimistic rather than as the truth. CMYK is where the specifics live: four named inks, a total coverage limit, and a gamut with its own shape.

A subtractive stack is also why several print techniques on this site are tricks about layers rather than choices about colour. Halftone breaks a tone into dots so that a single ink at full strength can stand in for every value between paper and ink, since the ink itself has no lighter setting. Duotone gets a whole image out of two inks by handing one the shadows and one the midtones, which only works because the two subtract in a predictable order. Both are answers to the same constraint: there is one paper, the inks only darken, and the lightest thing available is to not print.

Screens work the other way round, by additive colour, where three lights sum towards white and stacking translucent layers makes them brighter. Holding both models in mind is most of what it takes to predict which colours will survive the trip from one to the other.