The Collaborative International Dictionary of English v.0.48:

Metachrosis \Met`a*chro"sis\, n. [NL., from Gr. ? beyond + ? a
   coloring.] (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. --Cope.
   [1913 Webster]