Dark branchy chromatophores are perfectly visible inside the bodies of youngsters lately out of the larval stage. Chromatophores are the special organs that contain pigment grains of various colours.
Under the influence of hormones the pigment is either spread to all processes of the chromatophore cell making animal dark, either gathers all grains at the centre, making body transparent (just like this crab is).
Some crabs and prawns are able slightly to change their colour and tone adapting to the environment by dispersing grains of particular colour while concentrating the other.
In addition, the pigmentation of crustaceans often depends on the illumination (it was dark inside the cap, so crabs have got light), on the tidal cycle and even on the space radiation intensity.