Most of the paints that are available for use on the brass doorknobs and handles are often available as spray paint. Similarly, you can opt to use acrylic enamel paint especially if you are on a budget.
When doing the same, use a primer and then remove the handles using a screwdriver. You need to have a newspaper-covered surface, use a brush to paint the handles and plates once cleaning the same with a lacquer remover. After this, use grade 0000 steel wool and then remove the existing coating and then give a fresh polish on the brass surface.
Once done, use a wipe to clean up the same. This should be followed by using 2 or 3 coats of paint and this should be left to dry. Ideally the same should be left to dry for 48 hours before replacing back on the door.