The shell of the crustaceans is normally blue-green, grey or brownish, with only a slight tinge of red. When cooked, they take on the typical red lobster colour.

The skin armour (exoskeleton) consists of pigments, including a carotenoid called astaxanthin, which provides the reddish tone. At normal water temperatures and when the lobster is alive, the astaxanthin pigments are not visible and are covered by other protein layers. These protein layers are greyish, brownish or olive green in colour.

The heat of the cooking water destroys the upper protein layer, while the astaxanthin underneath remains heat-stable. After the top layer is destroyed, the red pigments remain, giving the lobster its familiar red-orange colour.