The rings are a confession the animal would rather not make. Most of the time it is invested in the opposite effect. Chromatophores, the elastic sacs of pigment under nervous control, change the tone of the skin to match sand, rock, or weed. Papillae change the profile, so the outline is not the smooth bag a predator is looking for. The body, having no bones that matter for this purpose, narrows through gaps and settles into them. A blue-ringed octopus in camouflage is doing the standard octopus job, and doing it well enough that the famous color can stay in reserve.
Den maintenance is the other half of not being seen. Observers have watched these animals, like many octopuses, pull stones, shells, and bits of rubble across the entrance of a hole. It is a small wall. It breaks the dark circle of a hiding place, and it puts a little work between a predator and a soft body. It is also, if you are learning to read a shore, one of the kinder clues you can get. A tidy little barricade in a crevice means someone is home. It does not mean you should open the door to check.
Camouflage has a limit, and the limit is why the rings exist. A predator that has already found the animal, or a hand that has already closed around it, is no longer going to be fooled by a sandy color. The switch to yellow and blue is the admission that hiding has failed. Understanding that sequence keeps the two looks from seeming like two different creatures. Drab and brilliant are the same octopus on either side of being discovered.
Photography can live with this. A long lens, a still body, and no rearranging of the stones will get you the resting animal, which is the rarer picture and the truer one. The full display is what you get after you have already bothered it. You do not need that picture.