Many species of octopuses (and some other cephalopods) have extremely efficient mechanisms for camouflaging themselves against coloured backgrounds. They also use coloured patterns for communication and for repelling predators. The physical mechanisms for generating the colours in their skin is well understood (see Chromatophore at Wikipedia).
Microscopic examinations of the octopus eye, however, have shown that they don't have the necessary retinal cells (photoreceptors) to perceive colour. Also, laboratory experiments which have attempted to test their vision skills have found, in all cases, that they appear to be profoundly colour-blind.
Raising the question of how they can camouflage themselves to precisely match a background - the colours of which they can't perceive. The system also appears to work in extremely low light conditions.
The extreme range of coloured displays which they use to communicate with each other is also unexplained given that the recipient apparently can't see the colours.
A theory presented in PNAS vol. 113 no. 29, 2016, suggests that they might be using chromatic aberration (i.e. colour separation caused by diffraction) caused by their unusual slot-like pupil shape. Source
Other theories suggest that the octopus skin itself may somehow be able to perceive colours, but the photoreceptors which have been found in the skin also appear to be monochromatic.
“ - the photoreceptors found in an octopus’s skin are, like those in its eyes, insufficient to detect colour. The best working hypothesis is that some complex interaction between the skin’s photoreceptors and chromatophores allows the octopus to adopt colours it cannot see.”
Amia Srinivasan,London Review of Books , Vol. 39 No. 17, September 2017.
Further reading (with photos) : Cephalopod dynamic camouflage in Current Biology, Volume 17, Issue 11.