Hold a magnifying glass to the screen you are reading this on and you will find a small disappointment: there is no yellow. There is no orange, no brown, no teal, no magenta. There are only tiny red, green and blue lights, arranged in stripes, switching on and off at brightnesses you cannot resolve. Everything else — every sunset and skin tone and this very teal accent — is a confection your screen builds from just those three. The screen is, in the most literal sense, lying to you about colour. It is also doing precisely what it was designed to do.
The mechanism is additive colour mixing, and it is the opposite of the mixing you learned with paint. Paints are subtractive: each pigment removes wavelengths from white light, so the more you combine the darker and muddier you get, until everything collapses towards black. Light works the other way. Each source adds wavelengths, so combining them grows brighter, and red plus green plus blue at full strength does not make mud — it makes white. This is why a stage lit by coloured spotlights and a canvas covered in coloured paint behave like sworn enemies.
Below is the whole trick, exposed. Three sliders feed three overlapping circles of pure red, green and blue light. Push them around and watch the overlaps: red and green light combine into yellow with no yellow anywhere in sight; all three together pile up into white in the centre. The live swatch and hex code show the single colour your screen is currently faking out of nothing but those three primaries.
Here is the part that turns a neat demo into something genuinely strange. When you slid red and green up and saw yellow, your screen did not produce any yellow light. It produced red light and green light, side by side, and your eye did the rest. Real yellow light — a single wavelength around 580 nanometres, the kind a sodium street lamp emits — would excite your eye’s red-sensitive and green-sensitive cones in a particular ratio. Red light plus green light, two entirely different wavelengths, happen to excite those same cones in the same ratio. Your brain receives identical signals and cannot tell the two situations apart.
That phenomenon has a name — metamerism — and it is the loophole the entire display industry lives inside. Two physically different mixtures of light that produce the same cone response look identical to you, full stop. A screen does not need to reproduce the true spectrum of a banana; it only needs to fool three types of cone into firing as a banana would make them fire. With just three primaries it can match a vast range of real colours, not because it recreates them, but because your eye only ever sampled the world through three channels to begin with. The screen isn’t cheating the physics; it’s exploiting the receiver.
Why three, and why these three
The screen has three primaries because you have three cone types, tuned roughly to long, medium and short wavelengths. A mantis shrimp, with around a dozen photoreceptor classes, would find your monitor a flickering, impoverished mess and your television unwatchable. Red, green and blue are chosen because their cones sit far enough apart in sensitivity to span the widest possible triangle of mixable colours — the device is reverse-engineered from the anatomy of the eye it is trying to satisfy, which is a quietly flattering thought to have about your own retinas.
So the screen is not lying so much as speaking your eye’s native language with a deliberately small vocabulary. It cannot show you a colour your cones can’t be tricked into; it can show you almost anything they can. The yellow you saw a moment ago was real to you and only to you — a private fiction assembled from red and green, convincing because the audience was built to be convinced. Which is, when you think about it, a rather generous arrangement: the universe declined to hand us a fourth cone, and we built an entire visual culture inside the three we got.