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.
Indeed, from an evolutionary perspective it is interesting that octopuses appear to be colourblind. Could it be that all the modern octopus families evolved from ancient deep-water forms who did not need colour vision?
Packard once suggested that the radiation of fishes and reptiles in coastal regions drove the chambered shell cephalopods into deeper and offshore waters. Chambered cephalopods, such as the ammonites and nautiloids, could not reach the deeper waters due to danger of implosion, and prompted the rapid evolution of the coleoids.
Perhaps a gradual recolonisation of shallower waters following the Cretaceous extinction by the colour-blind deeper-water non-shelled cephalopods drove animals that had lost their colour vision back up to repopulate recently vacated waters and fill a vacant ecological niche.
Granted we have the shallow-water Carboniferous octopod Pohlsepia and the Jurassic Proteroctopus, but there is precious little to suggest these were the direct ancestors of the Incirrata, perhaps all modern descendants split from their deeper-water cousins.
I still want to know about the degree of colour perceptivity of marine animals in general. Is colourblindness really as much of a disadvantage as it seems? I had assumed that many (shallow water) reef animals would have good colour vision, given the splendour of their environment, but since cephalopods were initially included in that assumption I'm not sure what to think. Perhaps I need to some more...
I've been looking for an answer to this question, too - how do they match such different colors if they are colorblind. I watched my bimac match different purples - red purple, blue purple, the colors of coralline algae. She couldn't have perceived only shades of gray and matched these colors. So how did she do it? And wasn't some research done recently that showed octos could distinguish red and blue?
That is a very good question and im happy to know the answer to.
Like Um... already stated, the color reflective cells on an octopuses skin surface aid the octo to camouflage in the wild through whatever environment they seem to pass by. However, generally...as stated in books and a wonderful guide called "The Ultimate Guide: Octopus" on discovery, the octopuses, cuttlefish and squid see only few tones of color in terms of being color-blind: Black, white...and shades of grey. Because of these colors, they think the pattern onto their brain, chance their color precisely and know exactly how to blend in with the different shades of greys wether it be plain grey, or darker greys. In general terms, theyan only see alternated color shades of grey...which can be white and black. Through our eyes, red is the color red. In their eyes, red looks like a soft-blackish shade. Instead of changing to red...mentally, they are changing their color and pattern to the blackish shade they see in their environmment. It's very interesting.
Another interesting fact is how octopuses can change different shades of colors when their chromatophores only have pigments of black, brown, red, and yellow. The timing of the contracting and expanding of these pigment cells gives the illusion of a lot more than that. Plus, they have billions apon billions of these sacks...which would surely and easily create illusional patterns of tones not yet seen in nature. I love when they turn black. Beautiful camoulflage. They have to be the masters of camoulflage. Able to alterate skin color, texture AND shape in a fraction of a second is incredible.
For me, the fallacy lies in thinking that one shade of gray corresponds to a particular color. I've even had courses in color theory and have also been interested in Ansel Adams photography (he discussed the gray scale a lot). A particular value of gray in a black and white photo might be green or blue or brown in the actual scene. It doesn't correspond to a single color. I have yet to see a good explanation for this part of the theory.
And this doesnt explain how a blind octopus can match its surroundings. Also I was under the impression that a severed arm would also match its' surroundings, not sure on either of these facts,(I read them somewhere) can anyone confirm if they are true?
Not too long ago someone posted about his octopus being able to recognize colors. He had 2 sets of plastic rings, one set inside the tank, the other set outside. He would hold up one of the rings, and his octo would find the same color ring and hold it up inside the tank. I believe he said tht the octopus could recognise 4 colors so far and he was working on training it to recognize a fifth. Not having seen those sets of rings, and I'm presuming that they are identical, I don't know if the rings have the same tone and intensity. I have some books on color, since I work with color in fiber arts, and do a lot of dyeing. Unfortunately they are all packed right now, so I can't access them, but Nancy is right that several different colors could all share the same tone of gray. Maybe we are making assumptions about octopus eyes based on inadequate information. There has to be something about the physiology of their eyes that make it possible for them to discern colors, even if our current knowledge would indicate color blindness.