The Oldest Trick in the Light Show

Your phone isn't showing you colours. It's whispering three of them at your eyeball very quickly and hoping you don't notice the difference. You don't. Nobody does. And the reason goes back a lot further than Silicon Valley.

What are cone cells?

The human eye has three types of cone cells, each tuned to a rough neighbourhood of the spectrum: one that responds mostly to red, one to green, one to blue. Not three precise notes, but three overlapping blobs of sensitivity. When real light from the real world hits them, thousands of wavelengths all arrive at once and the brain blends them into what you experience as colour. A ripe lemon sends a very specific shade of yellow to your cones. Your screen, on the other hand, has no yellow light in it at all.

Why do three colours make all the others?

What your screen does instead is fire a tiny red pixel and a tiny green pixel next to each other, so close together that your eye can't resolve them as separate. The two signals land on your cones at almost the same moment, the brain adds them up, and out comes yellow. Not real yellow. A very convincing argument for yellow. The distinction doesn't matter in practice – the brain has never been that fussed about reality, only about making useful decisions from the signals it receives.

Medieval Glaziers Knew This

This is, roughly, what the glaziers building medieval cathedrals understood long before anyone had named a cone cell. Stained glass windows don't glow because the glass is especially vivid. They glow because the eye is being asked to process transmitted light – light shining through colour rather than bouncing off it – which hits the cones with an intensity reflected surfaces rarely match. The craftsmen didn't know the science. They just knew the effect, and they built entire architectural experiences around it. The screen in your pocket is doing the same trick at sixty frames a second.

The technical name for what your display is doing is additive colour mixing. Red, green and blue light combine upward toward white. (Paints and inks go the other way: they absorb wavelengths and the more you mix them the closer you get to a muddy brown, which is why primary-school art was always so disappointing.) By adjusting the brightness of each tiny red, green and blue point of light, a screen can fake roughly sixteen million combinations. Sixteen million colours from three. The brain accepts the invoice without checking the maths.

For a long and genuinely surprising look at how the cost of producing colour has always lagged behind how convincing the illusion appears, The Wall Changed. The Price of Colour Didn't. is worth your time.

Where the Trick Runs Out

The limits of the trick are real, though. Screens struggle with very deep, saturated shades – certain blues and greens that exist in nature simply can't be reconstructed from three light sources without a wider colour space. That's why "HDR" and "wide gamut" displays are selling points now rather than standard. The cathedral glaziers had the same problem: achieving a true red was notoriously difficult and expensive, because the metal oxides that made red glass were rare. Some things stay awkward whatever century you're in.

Your eye, meanwhile, keeps accepting everything it's handed without complaint.