I know someone has already mentioned the Sally Lightfoot Crab, but I'm not sure if they meant the true Gallapagos Sally Lightfoot or the one I prefer, so I put it forward for your consideration:
Sally Lightfoot (Percnon Gibbesi)
http://www.ffexpress.com/images/inverts/crabs/20443.jpg
Dark green with nice light green markings. It would make a nice addition to all the red, yellow and blue stuff that's already there.