The famous photograph is the warning. The ordinary animal is harder to see, and it is the one that actually lives in the pool. Unbothered, a blue-ringed octopus tends toward tan, dull yellow, or a sandy brown. The rings are present as pale marks, easy to dismiss as mottling. A blue line through the eye may be the clearest clue, and even that can be subdued.
Texture fills in what color leaves out. Papillae, the little projections of skin, break up the outline and can make the mantle look like a nub of rock or a scrap of coralline algae. Arms settle into the grain of the sand. An octopus has no shell to hide behind, so the hide is the body itself, rearranged. This is the same toolkit other octopuses use. The rings are the specialty. The camouflage is the family trade.
People get into trouble in the gap between those two states. The animal under a rock looks drab, small, and handleable. The display comes after the rock has been lifted or the animal has been touched. Waiting to see the rings before deciding to leave is waiting until the warning is already a response to you.
If you are looking on purpose, and looking is a fine thing to do, search the edges: the lip of a pool, the underside of a loose stone you do not need to pick up, the mouth of a shell. A low yellow with a hint of circular pattern is enough. You do not need the full flash to know what you are seeing, and you do not need the full flash as a prize.