Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
For a Betta a bare bottom is OK, they are not bottom oriented fish. I would put some dark material under the tank (if it is not already sitting on a dark table or whatever)
Growing plants on driftwood or rocks will also be just fine with no substrate. However, as noted above there are not a lot of lodging sites for the beneficial bacteria, so use a large volume, slow filter. The plants that usually grow on wood are not really fast growing, either, so while they help, they are not fast growing nitrogen sponges. Clay pots make good containers for plants in a bare bottom tank.
None of that hides the dirt, though. Bare bottom will show the dirt even worse than sand.
To hide the dirt use a multi-colored substrate, perhaps mixed brown and tan shades, or add some soft grey or darker color to it. The assorted colors should be subtle, but will hide the debris until you vacuum. If you do not want to plant directly in it the substrate does not have to be very deep at all. As little as 1/4" is just fine for most sand-sized particles.