You have probably read about the chromatophore (color sacks) but these would be limited if it were not for the reflective iridophore and leucophores that really add to the color range and matching ability
It is fascinating to witness an octo mimic its environment, in both color and texture, so closely, especially knowing they are colorblind. It could be that they are matching 'brightness' and not color, or that iridophores are reflecting the wavelengths of colors around them (Mäthger and Hanlon 2006 found that chromatophore activity does not hinder polarization from underlying iridophores). There is much to be learned about the polarization sensitivity of cephs.
Only one species, that I am aware of, has been shown to have color vision (exhibiting three visual pigments as opposed to the singular rhodopsin in most cephs). This wonder of the sea is none other than the pelagic Watasenia scintillans, the "firefly squid".
You're absolutely right, Monty.
(I'm embarrassed. You sent me this article months ago - apparently I didn't retain the information very well! )
Here is an excerpt (pp. 693):
Most cephalopods that have been investigated have a single visual pigment with a lmax value in the range 470–500 nm (Messenger, 1981; Seidou et al., 1990). Mesopelagic species have lmax around 470–480 nm, as might be expected for maximum sensitivity to the down-welling daylight. There are some cephalopods, however, that possess two (e.g. the octopus Japetella sp. and the squids Pyroteuthis sp. and Pterigioteuthis sp.), or as many as three (e.g. the squids Bathyteuthis sp. and Watasenia scintillans), visual pigments (Kito et al., 1992). The best-known example is the remarkable mesopelagic firefly squid Watasenia scintillans (Matsui et al., 1988; Michinomae et al., 1994), which has a tiered retina of photoreceptors and three visual pigments (lmax=470, 484 and 500 nm, each based on a different chromophore: A4, A1 and A2, respectively). The 484 nm pigment is found throughout the retina, whereas the other two are found only in the ventral region of the retina. With three pigments present in the ventral retina, the firefly squid has the potential to make complex colour discriminations, which may prove useful in the spring, when it comes to the surface for communal spawning. This activity involves intense bioluminescent displays from a battery of photophores covering the body and tentacles.