How it works · 06
Colour
Why three colours are enough, why light and dyes mix in opposite ways, and why a white shirt looks orange under a light bulb in a photograph but not to the eye.
Try it: mixing colour, and the colour of light
Three kinds of cone
Newton showed in 1704 that white light is a mixture of all the colours of the spectrum. The eye, though, does not measure the whole spectrum. It has three kinds of cone cell, most sensitive to long (reddish), medium (greenish) and short (bluish) wavelengths, and every colour we see is a mix of their three signals. Thomas Young proposed this around 1802 and Helmholtz developed it: the trichromatic theory. It means a picture does not have to copy the spectrum. It only has to produce the same three signals, and three well-chosen primaries are enough.
Adding light
Shine red, green and blue lights on a white wall and they add: red and green make yellow, all three make white. That is additive mixing. Maxwell’s tartan ribbon of 1861 was made this way: three black-and-white photographs through red, green and blue filters, projected through the same filters and overlaid. The autochrome packed millions of tiny dyed starch grains, a mosaic of additive filters, onto one plate; the Bayer filter does the same on every digital sensor, and every screen shows colour by adding red, green and blue light.
Subtracting with dyes
A print or a slide cannot add light; it can only take light away from white. Each dye absorbs one primary: cyan absorbs red, magenta absorbs green, yellow absorbs blue. Overlap all three and nothing comes through: black. Ducos du Hauron described both methods in 1868–69. Modern colour film stacks three emulsion layers, each sensitive to one primary, and development forms the matching dye in each (kodachrome, agfacolor neu, colour negative film).
The colour of light
Light sources are not the same white. A hot object glows red, then yellow, then bluish-white as it heats, and photographers describe light by the temperature of a glowing body that would match it, in kelvin: about 1900 K for a candle, 3200 K for a tungsten lamp, 5500 K for midday sun, 7000 K or more for open shade under blue sky. The eye adapts and sees white paper as white under all of them. Film does not: daylight film turns orange under lamps, which is why tungsten-balanced films and blue correction filters existed. A digital camera corrects in software, with a white balance setting that scales the red, green and blue channels so that something neutral comes out neutral; a raw file lets you choose it later. See colour temperature.