as stated depends what you arekeeping and also the look you are going for
having trawled this forum for a month or so looking at members tanks
i have noticed some tanks look better with sand , coloured gravel or natural gravel
also you can use small or large gravel
personaly i like small dark gravels for effect as i have albino cory's and guppies the fish show well against black and it sets off my mopani rather well also
Piccies soon once camera fixed
so personal preference and fish welfare play a big part in what you use