How do chameleons change color?

The answer

Chameleons change color by actively tuning the spacing of nanocrystals within specialized skin cells called iridophores. When a chameleon is relaxed, these guanine nanocrystals are closely packed and reflect shorter blue wavelengths; when excited, the crystals spread apart and reflect longer wavelengths like reds and yellows. Beneath the iridophore layer lies a deeper layer that reflects near-infrared light, helping with thermoregulation. Color change serves multiple purposes including social signaling, temperature regulation, and camouflage, though communication with other chameleons is now considered the primary function.