gravel would probably be better because of the cleanup. Its easier with gravel. Black would probably be good for p's. There would be a good color contrast, and they seem to like it a litter darker ime
I have black sand.
It's expensive but it is awesome.
I literally opened the bag and tossed it in.
Then filled it up with water.
There was some floating **** but I just used my net and skimmed it off and it was GREAT.
No clouding what-so-ever.
I use it with my canister filter with no problems.
I don't even turn my canister off when I get to do water changes.
When you siphon it, I barely get any that siphons actually out of the tank.
It falls right back down like the gravel does.
I say black substrate make your fishes really stand out especially the coloration. Also if you know how to set it up, your tank would look Awsome. But the only thing is that you can easily see waste (poop) on the surface. But you can mixe with some white sand to camoflage?? (spelling??) it. That would really help.