I cannot think of anything you can do easily that is not either risky or a bad idea. You could expose the cue to a lot of direct sunlight in hopes that the finish will yellow. Needless to say this would be risky, as it could cause cracking, swelling, or warping. A cue maker who uses UV curing could spin the cue and over expose it with their UV light and it might yellow. Beware of this one as some finishes can actually shatter like safety glass when over exposed to such conditions. So that is also risky. A coat of finish sprayed over it that had some tint in it would work. This would change the diameter and may cause some fit problems. So I consider this one a bad idea. A complete strip, stain and refinish is the best option. Is is worth the expense? Only you can decide.