For those of us with them darn PIA white-lettered tires I've found the perfect solution for keeping them white. I tried all sorts of things including sandpaper, Brillo pads, and those green abrasive scrubbing pads. I purchased this brush at Home Depot in the paint department. The reason I like it so much is that you just lightly graze the surface of the letters, no hard rubbing necessary, either before (best) or after you've applied your favorite tire dressing.
FYI: I generally start by giving my tires a thorough cleaning with Westley's “Bleche-Wite” and a large nylon scrubbing brush, then go over the white lettering with the brass brush. Finally, I apply a light coating of Top-Of-The-Line “Best Dressed” vinyl & rubber dressing concentrate using a foam pad. Nice thing about “Best Dressed” is that it's water-based, dries completely so it doesn't pick up dust and grime, and it isn't a solvent that turns black as you apply it. All tire dressings seem to wear off in a heavy rain and this one is no exception, but I find that all I need to do next carwash time is clean the tires and wheels with a wet, soapy sponge, then reapply the dressing. Touching up the white letters is a cinch with the brass brush!
~Rich in NEPA~