There are two pigments in a crustacean’s shell, one is astaxanthin (and it’s the one that makes things red or orange,) another is called “crustacyanin” and it makes things blue or grey.
The astaxanthin is bundled up in the crustacyanin, so while the crab is alive, you see the shell as being bluish-grey. But when you cook the crab, the crustacyanin breaks down and that’s what causes the colour change.
Incidentally the reason flamingos are pink is because they absorb astaxanthin from the prawns that they eat.
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