Tried armour-all?

Sounds wierd but worked OK with mine. The main outer cartridge of mine were starting to look quite grey, gave them a hit with it and while not perfectly black (they never were from new) again it certainly looks better than a coat of paint. The armour-all seems to draw into the plastic (I have no idea why, it just seems porous). You may be able to use some trim dye but have seen too many people use it in high traffic areas where it will eventually need re-treating.

Dave