How Many Colours Are There, Really?

How many colours there are depends entirely on what you are counting. A rainbow has seven colours by convention and about six by eye. A typical person can tell somewhere around a million colours apart. A standard screen can display about sixteen million. And the light itself contains infinitely many wavelengths, because wavelength is a continuous quantity with no smallest step. Every one of those numbers is correct for its own question. None of them is the answer to "how many colours exist", because colour is not a thing in the world with a count. It is something eyes and brains do with light.

• A rainbow has seven colours by convention and about six by eye.
• A typical person can tell roughly a million colours apart.
• A standard screen shows about 16.7 million; a printer fewer; English names eleven basic colours.
• Light itself contains infinitely many wavelengths, so there is no final number.

Here is what each number means.

Seven if you ask Newton, six if you ask your eyes. Red, orange, yellow, green, blue and violet are the bands most people can pick out. Newton added indigo between blue and violet because he wanted the spectrum to have seven divisions, like the seven notes of a musical scale, and the list stuck. In reality a rainbow is a smooth gradient with no bands at all; the bands are where your brain draws lines between colour names. The no purple in the rainbow article explains why the gradient stops at violet and why purple is missing.

Infinitely many. Visible light runs from roughly 380 nanometres at the violet end to about 700 at the red end, and between any two wavelengths there is always another. There is no smallest step, so there is no count. This is the sense in which colours are infinite, and it is also slightly misleading, because two wavelengths a thousandth of a nanometre apart look identical to any eye that has ever existed. Infinite wavelengths do not mean infinite colours you could see.

Estimates range from about a million to ten million, and around a million is the commonly quoted figure. The number comes from three cone types, each able to register a few hundred levels of brightness, combined into a three-dimensional colour space. Multiply the steps along each axis and you get millions of distinguishable points. A few people, mostly women, carry a fourth cone type and may distinguish more; people with colour blindness, missing or shifted cones, distinguish far fewer. The colour blindness article covers what a two-cone world looks like.

About 16.7 million on a standard display, which is 256 levels each of red, green and blue multiplied together. Newer displays with ten bits per channel reach over a billion. That sounds like more than the eye can see, and in one sense it is, but a screen's colours all sit inside a triangle defined by its three primaries, and the most saturated colours in the real world, a spectral violet or a laser green, fall outside it. A screen shows many colours and not all colours. The additive versus subtractive explainer sets out why.

Fewer than a screen, and from only four inks. Cyan, magenta, yellow and black, printed as tiny overlapping dots, reproduce every colour in a printed photograph, but the reachable range is smaller than a screen's because pigment mixing always dulls. That is why a photo often looks a little flatter on paper than on the phone that took it. Three subtractive primaries plus black is enough for millions of colours and not enough for the brightest ones. The three colours that cannot be mixed explains the choice of inks.

Eleven basic ones in English: black, white, red, green, yellow, blue, brown, orange, pink, purple and grey. Almost every language has a small set like this, and they appear in a surprisingly consistent order as languages develop, with black and white first, then red, then green or yellow, then blue. Beyond the basic set, English has a few thousand colour words, most of them borrowed from objects: lavender, teal, mustard, coral. A paint company can name a few thousand more. Naming is not counting, though. A colour with no name is still a colour.

That colour is a relationship, not a quantity. Light has infinitely many wavelengths. An eye with three cone types collapses them into a space with roughly a million distinguishable points. A screen or a printer samples a triangle inside that space. A language draws a dozen big regions on it. Ask "how many colours" and you have to say which layer you mean. There is no layer at which the number is simply true.

With overlapping filters in daylight. A CMY Cube carries cyan, magenta and yellow on one object, and as you rotate it in a window the overlaps shift continuously: green to teal to blue to violet to red to orange to yellow and back, with no steps anywhere. You are watching a continuous slice of colour space, the same thing a rainbow shows but under your control. A collection like the Collector's Pack multiplies the overlaps across three Platonic solids, and Plato's Cubes puts thirty-five cubes in seven spectrum colours in front of you, which is about as close to a hand-held rainbow as an object gets. The science page goes through the wavelengths.

Seven colours in a rainbow by convention, six by eye. Around a million a person can tell apart. About sixteen million on a screen, fewer on paper, eleven with basic names in English, and infinitely many wavelengths in the light itself. Each count answers a different question. Colour is what eyes do with light, and there is no final number for how many colours that makes.

Hold a continuous slice of colour, and step into a world of colour and curiosity.