Three types of pigment cells, called chromatophores, work together to make a frog green.

The chromatophores stack on top of each other. Melanophores make up the bottom layer. They contain melanin, a pigment that appears dark brown or black. Melanin also tints human skin.

On top of these cells sit iridophores.  Although iridophores don’t actually produce pigmentation, they reflect light off of the surface of purine crystals inside the cells. When light hits these cells, they produce a silvery iridescent reflection in frogs, as well as other amphibians, fish and invertebrates.