Additive and subtractive mixing

Cyan ink on white paper absorbs red light, so cyan and magenta printed on top of each other leave only blue.

Two ways to make colour

A screen starts dark and adds light. Paper starts white and takes light away. These are the two kinds of colour mixing. In additive mixing, used by screens, projectors and stage lights, each source adds its own light, and more sources make a brighter result. In subtractive mixing, used by inks, paints, dyes and coloured filters, each layer absorbs part of the light that would otherwise reach your eye, and more layers make a darker result. The OpenStax neuroscience text shows both side by side: red, green and blue circles that add towards white, and cyan, magenta and yellow circles that subtract towards black.

Why the ink primaries are cyan, magenta and yellow

Think of each ink as a filter that removes one of the screen primaries from white light. Cyan removes red and lets green and blue through. Magenta removes green and lets red and blue through. Yellow removes blue and lets red and green through. Now stack them. Cyan and magenta together remove red and green, leaving blue. Magenta and yellow remove green and blue, leaving red. Cyan and yellow remove red and blue, leaving green. All three remove everything, leaving black, at least in this ideal model. The secondary colours of one system are the primaries of the other, which is why the two diagrams look like mirror images.

In numbers, the model multiplies. White paper is 255 in every channel. An ink at strength s keeps a fraction 1 − s of its channel. Cyan at full strength turns the red channel into 0. Cyan at 50% turns it into about 128, giving rgb(128 255 255), a pale cyan. Stacking inks multiplies their fractions, so the order does not matter. Light works by adding instead: two lights at red 255 and green 255 make rgb(255 255 0). Drag the sliders and compare the hex codes in the panel.

Why printers add black

Real inks are not perfect filters. Each one absorbs a little of the light it should pass and lets through a little of what it should block, so cyan, magenta and yellow printed together usually give a muddy dark brown, not a clean black. Printing three layers of ink to make black text is also slow and wet. That is why printers add a fourth ink, black, and the system is called CMYK. The OpenStax text describes it as cyan, magenta and yellow plus black, the usual system for printing inks. Paints are messier still, because pigments scatter light as well as absorb it, which is why mixing paint never follows the clean model exactly.