Additive colour

Every screen a design is shown on makes colour by adding light. A pixel is three emitters, red, green and blue, and the colour a reader sees is whatever those three sum to: red and green make yellow, green and blue make cyan, blue and red make magenta, and all three at full strength make white. Nothing is being mixed in the sense a painter means. Light is landing on the eye and the eye is adding it up, which is why the model is called additive and why the three primaries are the ones they are: they are what human colour vision has receptors for, not what happens to be in the paint box.

Three consequences follow, and all three matter more than the physics does. A screen cannot show a colour darker than its own black, because black is the absence of emitted light and the panel has a floor: whatever it leaks when every channel is at zero is the darkest thing a design can specify on it, which is why a true-black theme looks different on an OLED panel than on a backlit one. Two translucent layers stacked on a screen get lighter rather than muddier, since compositing them adds their light together, and a lighten or screen blend mode is that addition made explicit. And a designer who learned mixing with paint has to unlearn it: yellow and blue give a pale grey on a screen, not green, because those two lights already contain all three primaries between them.

The model is the principle rather than any particular set of numbers. sRGB is a concrete space built on top of it, with its own three primaries pinned to real coordinates, a white point, and a transfer function between the value written and the light emitted, and every other RGB space is another set of choices about the same three additions. Getting the principle right is what makes a gamut argument legible: a wider space moves the primaries further out so their sums reach further, and no amount of moving them lets the sum of three lights be darker than none of them.

The opposite arrangement is subtractive colour, where every layer can only take light away, and the two are worth holding in the same head. Ink stacks towards black, light stacks towards white, and almost every difficulty in getting a screen design onto paper is one of those two facts refusing to be the other.