Many species of reptile have color changing ability; the most well know are the Chamaeleonidae, or Chameleons. Among the most colorful species are Panther chameleons (Furcifer pardalis) and Veiled Chameleons (Chamaeleo calyptratus). read more
Some iguanas, some anoles, bearded dragons, I think the phrynosomid lizards have some ability to shift tone. Now, if you want to see true colour-changing for camouflage, you need to look at the cephalopods. Squid, octopuses, and cuttlefish have some amazing colour changing abilities. read more