Yes, laikadog, it appears that many of them are colourblind. As far as I know, every species of octopus examined so far has been found to possess only one visual pigment. Furthermore, behavioural experiments and electroretinograms (which measure the response of the retina to a flash of light) also indicate that octopuses can't distinguish colour. Watasenia scintillans
, the Japanese firefly squid, is the only cephalopod I know of that is thought to have colour vision. Still, it might be a little premature to make too many sweeping generalizations.
Your second question is a very good one. Chromatophores
come in only a few colours (black, brown, red, and yellow) and the nervous system does not seem to be wired in such a way that an octopus can consciously control which individual chromatophores to expand and contract. Instead, it seems that cephalopods create their patterns by combining several basic 'preset' patterns of colouration. Many cephalopods also have reflecting cells known as leucophores
and iridophores. Leucophores reflect the predominant colours of the environment, appearing whitish in white light, bluish in blue light, etc., while iridophores produce blues and greens (through interference or diffraction--I can't recall which). Together, these reflecting cells might aid camouflage simply by passively reflecting the general tones of an octo's surroundings. Perhaps the general success of camouflage is a bit of a statistical thing, where cryptic responses only need to be good enough on average to allow species to avoid too much predation or get just a little bit closer to prey. This article has tons of information about chromatophores and patterning in cephalopods.
Good questions, and I hope they get a bunch of good responses.