Chameleons are famous for changing color. Many people think they mix colors in their skin like paint. But a 2015 study found something different. Chameleon skin has very tiny crystals inside.

When the chameleon changes the space between the crystals, the color of the light they reflect changes. Also, chameleons do not change color mainly to hide. They often change color to send messages to other chameleons.

Most people believe that chameleons change color by moving pigments in their skin, like mixing paint. However, a 2015 study from the University of Geneva showed that panther chameleons mainly use structural color. Their skin contains special cells with tiny crystals made of guanine, arranged in a regular pattern.

When the chameleon is relaxed, the crystals are close together and reflect blue light, which mixes with yellow pigment to look green. When it becomes excited, for example when it sees a rival male, the spaces between the crystals widen, and the reflected light shifts toward yellow, orange or red. Another study found that chameleons mainly evolved this ability to communicate with each other, not to hide.

A calm panther chameleon is green, and an excited one turns yellow, orange or red. One might picture a painter mixing pigments. In 2015, however, Michel Milinkovitch's team at the University of Geneva reported that panther chameleons rely chiefly on structural color. Inside cells called iridophores, nanoscale crystals of guanine form a regular lattice known as a photonic crystal.

When the animal is calm, the crystals are tightly packed and reflect blue light, which combines with yellow pigment to make green. When it becomes excited, the lattice expands, and the reflected light shifts toward the warmer colors. The researchers reproduced this shift by soaking skin samples in solutions of different salt concentrations, which made the cells swell or shrink. A deeper layer of larger, irregular crystals reflects infrared light and may help prevent overheating.

Pigments still play some part, and how nerves and hormones control the crystals is not yet known. A 2008 study compared 21 lineages of dwarf chameleon. It was correlational, so it could not rule out camouflage entirely. But species capable of greater color change had displays that stood out more from their backgrounds, which points to social signaling as the main force behind the trait.