Primary Colours and Colour Mixing

About this topic

Light mixes additively: red, green and blue are the primaries, and combining them yields white, with secondary colours of cyan, magenta and yellow. Pigments mix subtractively: cyan, magenta and yellow absorb colours, and mixing them all gives dark. Screens use RGB while printers use CMY plus black (CMYK).

Red plus green light gives yellow, green plus blue gives cyan, and red plus blue gives magenta. The eye has three types of cones, most sensitive to red, green and blue, so mixing three lights fools the brain into seeing nearly any colour. In paint, yellow absorbs blue and cyan absorbs red, so their mixture reflects mainly green.

Worked example: a phone pixel showing yellow lights its red and green sub-pixels and leaves blue off. Zooming in with a drop of water or a magnifier reveals the separate coloured dots. A blue object under red light looks black because it reflects no red. Stage lights in different colours overlap to produce secondary colours on the performers.

Colour blindness occurs when one cone type is missing or changed, affecting about 8% of men. A common misconception is that red, yellow and blue are the true primaries for everything; that traditional set is an art approximation. Another is that mixing all paints makes white, when it gives a muddy dark colour.