I have always had good luck with BFG's and never had a white letter issue of them browning, but I stay on top of them too and keep them white. Seems like if they ever get that browning started, then it stains them pretty well. If I find anything, I will surely share. Also, I have seen and talked to others in the show fields thar have custom white letter kits on their tires from a company called TredWear. They offer several font types and height letter kits to say about anything you want them to say. Great for custom styles verbiage.
Good luck!