Octopuses are the ninjas of the cephalopodic world. We’ve previously seen their transformations, but never as radically as this one.
A fisherman hauls up the octopus and tosses him down on the deck. At the 0:26 mark, this orange octopus hits that deck. By 0:28, he’s turned light gray, like the color of the boat.
-via 22 Words
Octopi turn white as a terror reflex. The octopus isn't trying to mimic the boat, it's scared out of its tiny cephalopod mind at being hooked and then dropped into the bottom of a boat, so the first thing it does is bleach in fear. The video cuts off, but you can see it starting to turn back to a wan brown at the end. If they'd dropped it onto a blue boat, it still would've turned white/gray.