It could be surface dirt, or the paint discoloring over time; Is it uniform all over, even underneath? There are things to bleach white, but many are not safe for paint (like sodium hydroxide (lye) oven cleaner, do NOT use).

I'd say first try a ("car") wax designed for painted surfaces, if that doesn't work, a "cleaner wax" that has ever so much extremely fine abrasive to refresh the surface. Try talking to a professional paint supply for auto paints, they may have recommendations on cleaning products. Research first before using anything, even a wax, that might fog the clear coat surface if it has, and still try in an unseen area first, like under the bottom bracket.

EDIT: They make "fiberglass stain remover" that has oxalic acid in it, designed for white gelcoat on boats, I do NOT know if safe for paint.