Colorful Juveniles

After several days of much rain from tropical storm Isaac, the baby giant blue crabs (Cardisoma guanhumi) appear in droves here... Remember that they only get the boring pale blue when adults. When still in juvenile stages, they are really pretty in all rainbow hues: green, yellow, red, orange, purple, pink, black... Do rainbows have pink and black colors in them? Do you know??? If not, why not???