I looked with one up for you because I didn’t know.
Chameleons change their color by manipulating specialized cells — called chromatophores — that contain different colors of pigment. For example, when a chameleon wants to convey a particular mood or message, its brain sends a message to its chromatophores, which then move pigments around to change the chameleon’s color.
Recent studies have shown that chameleons also have a special layer of cells — called iridophores — under their skin. These special cells, which contain pigment and reflect light, are made up of hundreds of thousands of guanine crystals. Chameleons can relax or excite their skin, causing these special cells to move and change structure.