I began using light coloured natural river sand (mostly collected by myself) but recently I upgraded to Eco Complete, a black substrate, to help my plants. I am very pleased with the contrast and good looks of black substrate, so I adhere to jdhef and pinkfloydpuffer comments.
Like everyone else, I think black would really make your fish stand out. If not that, then some natural-ish colored gravel would look good (Like in my tank I chose a mixture of darker browns. I'm very pleased with how it looks).