I don't think that theres a "good color", it all comes down to preference. Most will say including myself black is best because it give a greater contrast allowing the fishes colors to stand out more.
I like black, it does bring out the colors but in my opinion its not natural. I use play sand from home depot, it looks nice, is real fine, and is very cheap! I have the play sand in my 75 and black sand in my two 25's
A white gravel can wash out a fishes color. An off white mixture is better. Black is sweet but shows everything.
Put in what you like, you gotta look at it every day and clean it as well.