metachrosis

from The Century Dictionary.

• noun Color-change, as that of a chameleon.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Biol.) The power of changing color at will by the expansion of special pigment cells, under nerve influence, as seen in many reptiles, fishes, etc.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun zoology The ability to change colour by the expansion and contraction of chromatophores