Both of the species I mentioned are basically monomorphic, however, the Pseudotropheus elegans males and females can be distinguished by color as they become a mature age.
Pseudotropheus elegans 'msuli' (yellow tail acei) males have more blue/purple on their dorsal fin than do females who typically have a completely yellow dorsal fin.
Pseudotropheus elegans 'ngara' (white tail acei) males are much more vibrantly colored, very black with a very blue face and vibrantly white tail, whereas females are more of a black/brown color with a light blue sheen to their faces and a more dull white tail.
Labidochromis caeruleus males and females are basically indistinguishable.