WRONG: Red, blue and yellow are the primary colours

At school, you were probably taught in art classes that the primary colours – the ones you use to mix all other colours – were red, blue and yellow. Sorry to break this to you, but your teacher wasn’t telling the truth.

Primary colours, the building-block hues, aren’t a property of light itself but a consequence of the way our eyes work. Cells in the retina respond to red, green and blue; accordingly, everything we can see is made up of combinations of red, green and blue light. (Some animals have a fourth type of cell that gives them a fourth primary colour, probably close to the ultraviolet range. Bees can’t see red, but they can see “bee purple”, which isn’t purple at all, but a combination of yellow and ultra-violet. All this begs the question of why bees don’t attend more raves.)

TVs work on this principle. Shine a red light and a green light together, for example, and you’ll see yellow where they overlap. Add blue and you’ll get white where all three combine.

So much for light. Now back to art lessons, or the “subtractive” system of colour mixing. In the subtractive system, you don’t add light to make new colours, but start with white then filter it. White light bouncing off red paint appears red because all other wavelengths have been absorbed by the paint. That’s why black surfaces get hot in the sun, because they absorb much of the light, then transmit some of that energy as heat.

Mix two paints together and the effect will always be darker than its constituents, because even fewer wavelengths will make it through the double filter. The three primary colours of the subtractive system (the “primary pigments”) are cyan, magenta and yellow (ie they are colours that can’t be made by mixing other colours, and together make up all other colours). Add cyan to magenta and you get blue. Magenta and yellow make red, and yellow and cyan make green. Most “full-colour” printing is composed of four translucent layers – C, M, Y and K (black). The K layer is only necessary because so much of the average page is type and because no one’s yet synthesised perfect primary inks.

The only reason Miss Johnson gave you red and blue with your yellow paints in art class was that they’re a lot cheaper to produce than cyan and magenta, and while it seems obvious that red and yellow should make orange, it’s less immediately apparent that magenta and yellow will make red.