Try using “Bleche-Wite Tire Cleaner“ in a small inconspicuous spot on the white portion only, dampen a soft rag with the cleaner & lightly rub it & rinse with water, I’ve use this to brighten up yellowed (white) signs...
I have done this too on white signs.. put it on , and let it sit outside in sun a bit too helps... then have a hose nearby and wash it off good.. but just use caution..
