The color-changing ability can be used either for camouflage or to communicate with other octopuses. Altering the position of muscles beneath the skin can also change the texture of an octopus’s skin. This skill can be used to achieve the appearance of such common underwater objects as seaweed or rocks!
And, yeah, we said “octopuses” not “octopi”. Turns out, “octopuses” is completely acceptable, and if we wanted to be really picky about it we could have also said “octopodes”