According to Alaska Hermit, hermit crab owners should refrain from painting or decorating their crab's shell, as it risks the health and even the life of the animal. Shells that have been painted prior to sale should also be avoided.
Hermit crabs bought in painted shells were forced out of their natural shell, causing stress that adversely affects them. Painting a shell can also cause injury to the crab, either by effectively "gluing" the animal into its shell or exposing them to toxic chemicals present in the paint. For optimum hermit crab health, purchase only those with natural, unpainted shells.