An online reference:
The greatest arrayof defence strategies occurs in shallow-water, bottom-living (benthic) octopuses in the family Octopodidae. Diurnal octopuses such as the common Day Octopus (Octopus cyanea), which emerge and forage during daylight hours on coral reefs, have developed exceptional camouflage capacities. They produce elaborate colour patterns and highly complex skin textures capable of matching a wide range of backgrounds from sand and reef rubble, through to spiked corals and seaweeds. Their skin changes almost instantaneously as they move over different substrates on the sea floor. Colour changes are carried out by small, elastic, pigment-filled sacs, known as chromatophores. A square centimetre of skin may contain hundreds of chromatophores, in up to five colours in certain species. Each chromatophore is surrounded by a ring of muscle fibres, all of which are under the rapid and coordinated control of the large optic lobes of the brain. As a backup defence, most octopuses also have an ink sac that produces highly concentrated black, red or brown pigment. Small amounts of ink are squirted out the funnel to produce either a dummy decoy or, in some species, a full smoke screen that can mask a volume of water up to several cubic metres, leaving predators chasing their own tails. The ink is also thought to dull the senses of the predator.

those 8 leggers can really transform almost as fast as they swim over their background

I think its especially interesting how they can make their skin resemble structure, that's like friggin' 3D transmogrification, we're talkin' pixar's inspiration here