Once they reach around three or four inches males will turn bluish/grey with darker blue stripes while females will retain the yellow with brown stripes.
Under the pic in the link is a drop down menu for images of male, female and juvies.
Great species, just thought I'd point out they are not a pairing fish, like all mbuna they should be kept in harems. This is especially important with a large aggressive species like P.crabro. While I, kj, and Ferris have all kept single specimens in mbuna communities with no problems, I understand they can be holy terrors when in groups. I've read quite a few accounts of them wiping out entire tanks on cichlid-forum, if keeping more than one they are really best suited in a species tank of 6ft or more.