It depends on the fish. IMO things like severums, chocolate cichlids, acara-types, and discus will keep darker bodies and show their stress bars more when they are over a dark substrate. I've had the best luck with a black painted background and pool filter sand which is light tan (not quite white). Their colors lighten up a bit over the light sand but pop against the black background.
Of course this will not affect albinos or fish like super red severums, midas/red devils, or fish that don't have a ton of dark pigment.