How we see color

A retina holds about 6 million cones of only three kinds, and every colour you have ever seen is a mix of how strongly those three kinds respond.

Colour starts as light

A red apple does not contain red. Sunlight that hits it carries every visible wavelength, from violet at around 400 nanometres to deep red near 700. The skin of the apple absorbs most of the short and middle wavelengths and sends the long ones back. Those long wavelengths reach your eye, and your brain names the result red. Change the light and the colour changes with it: under a pure blue lamp the same apple looks almost black, because there is no long-wavelength light for it to send back.

The CSS Color specification from the W3C puts this carefully. The colour of an object depends on how much light it reflects at each wavelength and on the light that falls on it. The colour of something that gives off light, such as a screen, depends on how much light it emits at each wavelength. Two objects with different spectra can still look exactly the same, and when they do, we say they have the same colour.

Three kinds of cone

OpenStax College Physics counts about 6 million cones in the human retina. There are three types, and each type is most sensitive to a different range of wavelengths: one to long (reddish) light, one to middle (greenish) light and one to short (bluish) light. A single cone cannot tell you the colour of what it sees. It only reports how many photons it caught. Colour appears when the brain compares the three kinds side by side.

That is why yellow light and a mix of red and green light can look identical. Pure yellow light excites the long and middle cones about equally. So does red light plus green light shone on the same spot. The cones send the same message, and the brain sees the same yellow. OpenStax also points out that there is no single, unique set of three primaries: other sets work too. Red, green and blue are simply a convenient set for screens because each one drives mostly one kind of cone.

What a screen does with this

Look at a phone screen through a magnifying glass and you will see tiny red, green and blue elements. The screen never makes yellow light. It lights the red and green elements next to each other, they are too small to see apart, and your long and middle cones respond as they would to yellow. Turn all three up and you see white. Turn them all off and you see black, or rather the dark glass of the screen. In the model above, the centre where all three lights overlap is white, and each pair makes a lighter colour than either light alone. That is the rule for mixing light: adding lights always makes the result brighter.

Because three numbers are enough to describe what the cones report, three numbers are enough to describe a colour on a screen. The rest of this course is about those three numbers: how to arrange them in a solid you can turn around, how to pick colours that go together, and how to check that a colour can be read against its background.