I like the look of fish with black gravel. THe light seems to reflect off of the fish and not as much the substrate underneath. .... one thing positive about the lighter substrate is that the fish's underneath side seems to get more visable light.
I like the look of black sand in the tank...but I have some fish over black now and it totally messes up there color. But it seems like the trimac doesn't change too much. I might try the black sand and see how it goes.