Clean the bathroom's surfaces with trisodium phosphate (TSP). Even if your bathroom is already sparkling clean, the bathroom surfaces may contain household cleanser residues that react poorly with paint. TSP does not react with paint, and it is the preferred cleanser for most painting prep work.
Stick painter's masking tape to your bathroom's trim or wall fixtures. Anything that you don't want painted should be covered with tape or plastic sheets. Even the best painters make occasional mistakes.
Prime the bathroom using a tinted primer. If your bathroom already features a very dark wall color, then you're in luck, and you don't need to prime the surface. For common white or light-colored bathrooms, tinted primer is a must. Without a tinted primer, you will waste time and money on several extra coats of brown paint. Rollers are good for main surfaces, while paintbrushes help you prime edges. Wait 24 hours for the primer to dry.
Paint the bathroom with brown oil-based interior paint. Latex paint is conventionally used on most home surfaces, but bathrooms are one of the rare exceptions. Oil-based paint holds up much better against bathroom moisture damage. As with the primer, use a roller for main surfaces and a brush to paint the edges.
Apply a second coat of brown paint after the first coat dries. With a tinted primer, two coats is probably all you need.
Remove the painter's tape after 24 hours.