Actually, some fish are stressed by the coloration of their substrates. I'm not necessarily talking about the light vs dark just the coloration. It's been a few years, but I remember an article about some wild africans with some like neon gravel. What's funny is that as far as rockwork goes they don't mind substitutions. I used to have a link with pictures of them using skulls/bones of hundreds of killed humans as shelters.I really don't think it matters, if you have beautiful healthy fish they look good in with light or dark substrate.