Chromatophores are cells that change pigments and reflect light and in
cephalopods (e.g. octopus, squid) can be rapidly changed by shifting pigments
and reorienting reflective plates in the cells using their muscles! Caribbean reef
squid (Sepioteuthis sepioidea) use their chromatophores for camouflage and
communication! Flashes of changing colors and patterns between squids are often
seen during courtship and sometimes these little squiddies can simultaneously
send different messages to squid on either side of it!

[Fact and sketch contributed by Karli Chudeau (@KarliRChudeau on twitter)]

Source:

Cloney, R. A., & Brocco, S. L. (1983). Chromatophore organs, reflector cells, iridocytes and leucophores in cephalopods. American Zoologist, 23(3), 581-592.