The 3" Dempsey I'm holding for a friend used to be almost completely black all the time while it was kept in my 185L/50G tank with three other Central Americans of similar size. Once it lost its place in the pecking order to a recently introduced convict it lost the black completely and looked like a regular Dempsey, albeit more colourful than average.
It's now in a 50L/13G tank with a couple smaller tankmates and it's darkened up a bit but the black hasn't returned.
