First clean it out as best you can. Dump out any left over wood and charcoal. Scrap any lumps of ash off the bottom. Hose the pit off. Once dry, go ahead and paint.
We suggest high-heat spray paint (may be labeled BBQ Paint) from either Tremclad or Rustoleum. Or, the canned version of paint is a lot thicker and will last longer, but of course is slower to apply.