How chameleons change colour: they stretch a mirror made of crystals

A chameleon doesn't change colour by mixing pigments. Under its skin sit cells packed with tiny guanine crystals, arranged in a lattice that works like a tunable mirror.

When a male panther chameleon gets excited, the crystals move about 30% further apart and the mirror starts reflecting longer wavelengths. Seen through a layer of yellow pigment, calm blue shows up as green, and the longer excited reflections as blazing yellow and orange. The flashy changes are mostly for talking to rivals and mates, not for hiding.