Why not get coloured fish instead. With natural gravel, the fish's colour will stand out and look brilliant.
The next time Doctor Dolittle comes for tea, I shall get him to ask my fish what they'd prefare. I'm pretty sure though, that they might prefare natural rather than 70's psychodelic minging colours.
A serious question, though.
Has anyone with a large tank ever tried to test fish preferance with gravel types?
I'm guessing you would need similar decor (rocks, caves and stuff) at each end of the tank so that choice is based on the different gravel colours at each end.