Usually when fish are the most colorful its because they're happy and healthy. Many of them color up when they're breeding. Some will color up best when their water quality is as close to 0ammonia, 0 nitrites, 0nitrates. You just need to get their environment as good as possible. If they're constantly stressed out because of the choice of fish you have together, then there's not much you can do to get them colorful except take away the cause of the stress.
If you feed them good quality food and mix it up you'll see good results....see which frozen foods they like best (bloodworms, brine shrimp etc) If they're big enough, let them hunt ghost shrimp every once in awhile.
The tank background and substrate (gravel) lend themselves to the overall atmosphere of the tank. If you have skittish fish, the darker the substrate and background and more subdued lighting will help them feel more secure and probably make them happier (ie, might help them color up more). For africans I'm not sure that they're real concerned about the color of their tanks.
Africans are usually very territorial unless you've gone with the overstocking theory to keep aggression down. Either way, I'd think that lots of caves and hiding places for your fish helps them feel more secure also.