They can go light & dark at will, so dont worry too much about how their colors come through on different substrates. As long as its white, black, or tan they will be plenty vibrant. Of course, the tanks lighting makes a huge difference, as well as their nutrition, number of hiding spots & the shoal size... So, I guess my point is get the sand color you like (yes, I'm suggesting sand) & let them surprise you with all the colors they can make.