How would I paint the brass on a pendant bar light?

Best way would be to remove the light. Lightly sand the brass to remove any gloss. Tape and paper off any part/parts you don't want painted. The spray paint the brass with metallic paint. Several light coats will get you a smooth finish. If you don't want to remove the the light, lightly sand the brass to remove any gloss. Tape and paper any part/parts you don't want painted. Lay down news paper under the lamp to catch any drips Then paint with metallic paint in a can. Use a sponge brush to get a smooth finish without brush marks.

I hung mine from a tree..preped it the way William said and sprayed it with a hammered nickel (Rustoleum) finish...it was beautiful...helped me sell old house

I spray painted a bunch of brass light plates with a Metallic nickel and they look great!

Remove fixture wash (maybe with TSP), dry, tape off any spots paint is not wanted., spray paint with a high quality spray paint. Let dry and Install. I suggest using a garden Shepherds Hook to hang the light on to spray paint (you can then carry all in shelter if weather requires). Another hint is to begin painting fixture upside down (this would be parts seen from the floor) first. I have had excellent results on light fixtures, lamps, and wall switch covers etc. Use quality spray paint.