Whether your brass fireplace doors have become dingy and tarnished, or you're just ready for a change, painting them is a fairly simple weekend project. Your doors may be solid brass or brass-plated steel -- a magnet will not stick to solid brass -- but in either case the process is the same. Taking the doors off will probably be a longer task than doing the actual painting, and aerosol spray cans will make quick work of the painting.
Remove the doors and frame. Most fireplace doors are mounted on a metal bracket; you may need a wrench or screwdriver to remove the doors and frame from the bracket.
Disassemble the doors as much as possible. If they have black chain or mesh curtains, these will probably be attached to a single bracket to the frame.
Clean the fireplace doors well with acetone and a fine sanding sponge. This removes sooty buildup and oil; using the sanding sponge to clean the doors will scuff the surface at the same time so the paint will adhere well. Wipe the surfaces with a clean, dry rag.
Mask off what you can't remove, such as the glass fronts. Use masking tape and newspaper to precisely cover the glass on both sides.
Lay the fireplace doors on clean dropcloths or cardboard.
Spray the doors with metal aerosol primer. There are several types of primer; the paint and the primer you're using must be compatible. Apply several light coats instead of one or two heavy coats, until the brass finish is obscured.
Apply several coats of paint in the same way you applied the primer.
Apply a coat of clear protective sealant for a glossy, scratch-resistant finish. Not all spray paints require this step.
Reassemble and install the fireplace doors after they've dried for 24 hours.