There are some 150 species in North America, over 540 species worldwide. Color and size varies with species, diet, and age. Most are red, some are green, brown, tan, or blue with black or orange markings in various combinations. Often juveniles will be a light tan color that turns to a deep red as an adult. The coloration depends in part on their diet, and can change with a change in diet.
Some Australian varieties can be much larger.
When they first molt they are soft, and very vulnerable to attack by other crayfish or fish. It can take a couple days for the new shell to fully harden. Juveniles can molt every week or so. Adults may only molt a couple times a year, and only under the right conditions. The empty shell should be left in the tank, where it will be consumed over the next few days to recover the lost minerals and help form the new shell.