My guess is since you are talking bikini top these are some type of canvas. I have seen canvas half doors and custom fabbed metal ones, so if they are metal, paint or powder coat as already stated. Read somewhere, maybe even on this board that RIT dye may work. I would make up the RIT dye in a small tub where the doors fit into flat. Make it just deep enough to cover them. Same for the top if possible so there are no fold lines. I would probably try this before using fabric paint, but it is available. Vinyl paint is always an option, I have used it many times in the past when I wanted to change the interior color of a car I had. My only concern with paint is that it would flake off as the top and doors flapped during higher speeds. MAybe even try a upholestry shop and ask them what works. Good luck, if you try the RIT dye let us know how it works.