Three Lights and Nothing Else

A screen cannot make colours. It has only three kinds of light per pixel — a red, a green and a blue — and it adds them. Every photograph, every film, every shade you have ever seen on a screen is those three, at different strengths, too small to tell apart. Drag them. Then zoom in.

Why it adds instead of muddying

On paper, red and green make a dirty brown. On a screen, red and green make a bright yellow. Same two colours, opposite result — because they are doing opposite things. A pigment works by subtracting: red paint is red because it swallows every wavelength except red, green paint swallows everything except green, and two greedy subtractors stacked together leave almost nothing. A screen works by adding: it does not remove light from a white page, it emits light into the dark. Pour red light and green light onto the same spot and you have more light, not less. That sum lands, for your eye, on yellow.

The yellow with no yellow in it

Here is the part that should feel strange. Set the sliders to yellow (red and green up, blue at zero). The screen is emitting a spike of red light near 610 nm and a spike of green near 540 nm — and nothing in the yellow band around 575 nm in between. There is no yellow light leaving the screen at all. Yet you see yellow, and you cannot tell it apart from a pure 575 nm sodium-lamp yellow. Your eye samples all of light through just three kinds of cone — three numbers — and two different spectra that produce the same three numbers are, to you, the same colour. The map below recomputes, live, which single wavelength each mixture imitates.

Drag a corner colour out to its purest and you hit the edge of the triangle — and the curved rim of real colours is always outside it. That gap is honest: a screen reaches only about a third of the colours your eye can see. The most saturated reds, greens and cyans in the world simply have no setting on these three sliders.