I have betta tanks with green gravel just cause I had it and it brightens up the small tanks. I have natural colored gravel in the 2.5G Paradise tank. I have black sand in a 6G that needs fish and/or a rebuild. The electric blue crayfish is on purple and black. It is kind of trippy, but he's a trippy critter.
When I did larger planted tanks (before I got the reef bug), I did natural colored gravel by itself or over dirt, or black sand.