well, aperantly the colour of the gravel/substrate/decor affects the mood of the fish because fish use colour to show thier mood. My own expierence with buenos aires tetras, wich one book even labels as viciuos and that the females can't be with the males because the females will kill the males is that they are viscous with flourescent pink gravel and perfectly peaceful, almost barely moving except at feeding time (there still agressive feeders) with mostly white gravel.
So, I guess the fish do care what colour there substrate is but don't care if there substrate is unnatural(niether whit or pink are natural colours for fish) or natural, with the exption of : Wild caught fish, you should try and make thier tank look as close as possible to thier natural enviroment, just because of what you've paid somebody to do to them, respect the fish. Here I find it for the for the most part cruel to put them in a tank with neon green gravel and purple plants or any other unatural decor or no decor at all, the fish tank is unatural enough.