Is there anything that can bring back the white color in a handle, after it has yellowed?
I haven't refinished a lot of handles so someone else with more experience will likely give you better advice. I've only refinished one white handle and high grit sandpaper followed by plastic polish brought it back to life for me. Again, it will depend on how much it's yellowed and how deep the discoloring is.
dfoulk nailed it. You'll need to take the yellowed surface off then progressively sand & buff it to the sheen you want. Or:
wouldn't be something if oxiclean actually worked!