Mark H
"Back in January I followed this octopus as it hunted. It explored a rock covered in yellow-green kelp, and as it climbed around, it sometimes produced a perfect match for the colors around it. It was hard to see where seaweed ended and octopus began." - Mark H from Bookmarklet
"How do octopuses trick the eye? One part of what they have to do is produce particular colors. This is done with several layers of specialized cells in their skin. The chromatophores, near the surface, contain sacs of pigment, and the sacs can be expanded or contracted by tiny muscles to reveal or conceal a particular color. The colors of chromatophores vary according to species, but the only available colors are black/brown, red, orange, and yellow. Greens, blues, and violets are produced differently, with the aid of several kinds of reflecting cells that lie deeper in the skin. These cells can bend and scatter light, as well as reflect it straight back." - Mark H
Science Friday: Where's the octopus? (video) - bentley