Get really close to a phone or TV screen and look hard at a single bright dot. It isn't one colour. It's three tiny lights crammed together: one red, one green, one blue. That's all a screen ever has. To make any colour, it just sets how bright each of the three lights is and lets them shine together. Your eye, from normal distance, is too far away to see them separately, so it blends, or adds, them into one colour. Red and green together make yellow. All three at full make white. All three off is black. Slide the red, green and blue lights in the simulator to mix a colour, then zoom in to see the three separate lights.
Most people think a yellow image on screen is made of yellow light. In fact the screen has only red, green and blue subpixels; it fires red and green together, which stimulates your eye exactly as real yellow would.
What's actually happening
Almost everyone assumes a screen makes the colour you see directly: that a yellow banana on screen is made of yellow light. It is not. A screen has exactly three kinds of light to work with, red, green and blue, and nothing else. Every pixel is a cluster of those three subpixels, and the only thing the screen controls is how brightly each one glows. There is no yellow lamp in there, no orange one, no purple one. Just red, green and blue, side by side, too small to make out from where you sit.
It works because of how your own eyes are built. Your colour vision runs on just three kinds of sensor, tuned to roughly red, green and blue light, and the colour you perceive is your brain's reading of how strongly each of the three is stimulated. So the screen plays a trick: to make you see yellow, it turns its red and green subpixels up and leaves blue off. That mix happens to stimulate your eye's sensors in the exact same pattern that real yellow light would. Your brain, following its own honest rules, reports yellow, even though no yellow light ever existed. These imposter mixes are called metamers, and your screen relies on them millions of times a second.
That three-light shortcut is why a handful of primaries can counterfeit a whole world of colour. Eight bits per channel gives 256 brightness levels each, and 256 times 256 times 256 works out to about 16.7 million possible mixes, enough to fool the eye into seeing smooth skies and skin. It is also why mixing light is the opposite of mixing paint: stack all three coloured lights and you get brighter, ending at white, while stacking paints only ever gets darker. And it is why magenta on your screen is a kind of beautiful fiction, a colour with no wavelength of its own, conjured purely by firing red and blue while green stays dark.
Three lights counterfeit a whole world of colour by exploiting your three eye sensors, so the screen never needs real yellow.
- 1Put a small drop of water on a phone screen showing a plain white image, and look through it like a lens, or just press your eye very close.
- 2You will see white split into rows of red, green and blue dots. White was never one colour, it was all three lights on at once.
- 3Now show a yellow image and look again: the blue dots go dark while red and green stay lit. Yellow is just red plus green, with your eye doing the blending.
Common questions
It turns its red and green subpixels up and leaves blue off. That mix stimulates your eye's sensors in the same pattern real yellow light would, so your brain reports yellow even though none exists.
With 8 bits per channel there are 256 brightness levels each, and 256 × 256 × 256 works out to about 16.7 million possible mixes, which is why colour pickers run each slider from 0 to 255.
Stacking coloured lights gets brighter and climbs toward white, because each light adds. Mixing paints only ever gets darker, because each pigment takes colour away.