I have had good luck with Flitz polish cleaning up older bows, and it does get rid of some of the yellow.
I do know that if the bow came from a home with a heavy smoker, some of the yellowing can be accounted for by that. I am thinking that once it has yellowed, it has yellowed and not much can be done, so am afraid the only way to get back the original colors will be to refinish, or have it refinished by bowdoc or droptine.
RonP