A medium-to-dark gray is usually the most flexible single backdrop choice.

In a small room with white walls, it’s generally easier to make a gray background go darker than it is to make it go pure white. For low-key shots, control spill with flags (such as tall foam board), cover windows, and keep as much distance as possible between subject and background. That helps prevent your subject lighting from brightening the backdrop.

For brighter or colored backgrounds, you can light gray paper separately with a flash, including a gelled flash, to shift its tone and color. The main limitation in your room is likely light control and flash power, not the gray paper itself.

Since your room is already white, you can also use the walls themselves for high-key looks and reserve the backdrop for darker or colored setups.

A good practical test is to place an 18% gray card where the background would be and see whether your setup can push it convincingly dark and bright. If brightening it enough is harder than darkening it, choose a lighter gray; if darkening is harder, choose a darker gray. In many small home studios, darkening is easier than making gray look white.