How Does a Colour-Blind Animal Match Colour?

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An octopus can match the colour of a rock it is sitting on, in under a second, with skin that has no colour receptors, using eyes that are almost certainly colour-blind. Nobody has fully explained how. It is one of several things about this animal that do not resolve.

From the video

Colour-blind camouflage
Colour-blind camouflage
How Does a Colour-Blind Animal Match Colour?
Three questions
Three questions

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How Does a Colour-Blind Animal Match Colour?