In the video above, The National Geographic explains chromatophors, the pigment-changing cells in certain species of octopus.
The blue-ringed octopus is venomous and entirely willing to let you know before you get too close. Karel Mestdagh shot this video on a Samsung Galaxy S4 in a waterproof case: https://www.youtube.com/watch?v=3Bt1LvpZ1Oo
Octopuses' camouflage is a behavioral as well as colorful performance, mimicking other sea creatures from lionfish to cuttles: https://www.youtube.com/watch?v=IgDGTLRcwa8 https://www.youtube.com/watch?v=123RGfEWhI4
