White sand with a black background is one of the coolest set ups I have seen. Makes for a very bright tank. on the other hand white gravel may be the worst looking substrate I've ever seen, it has a real fake look to it.
For RBPs alot of people recommend black gravel, this does seem to bring out the red a little more but it make the rest of the fish dark too. I was getting ready to experiment with some different gravel colors for my RBPs, but ended up using them in a trade for a Black aro.