[cprocess]

If the knobs are brass plated and in good shape (not pitted, no corrosion), you may be able to plate directly over the brass. It is probably coated with a lacquer that would need to be removed, the brass then cleaned, reactivated with an acidic solution, then replated. From here, you have many options. For a darker look, there are cupric black oxide solutions that will give you different oxide shades including black, depending on time and temperature of oxidizing solution. You can then lightly relieve some of the black to give an antique look, then lacquer to protect. You could also plate a matte nickel, followed by black nickel to give a dark nickel coating. For a pewter look, I have plated parts in a matte tin solution, followed by the same oxide solution to give a gray pewter look. This would need a thin coat of lacquer to protect from further oxidation as well.
You can probably purchase these processes from Caswell and do it yourself for a fun home project. If you do not want to be adventurous, reply with your email address and I will process them for you.