I used black sand before and feel with darker substrate/background their colors pop more. Light substrate they still look nice but sometimes colors don't pop or look as dark. I believe they try and adept to their surroundings to hide from bigger predators.
The black gravel should be fine. Personally now all I do is bare bottom tanks. It depends too how nice their colors will be based on diet/genes/clean warm tank. If you feed shrimp or prawns make sure to dice them up really good and not give small chunks because it will cause bloat and can ruin their digestive system. I only give once a week