You don't have to resort to harsh chemicals, which may damage the rubber and/or the wheel. When I had these tires, my solution was to wet-sand the tops of the letters only. Use 240 grit on a rubber sanding block and plenty of water. You remove a microscopic layer of discolored rubber, and they stay white for months. Armor-All and almost any other "preservative" tends to worsen the discoloration problem, so try to stay away from it.
