They don't even have 100 miles on them and the car sits in the garage, so no sunlight. I tried a few things but what worked best was brake fluid.....I only scrubbed the letters and tried to keep it off the rest of the tire. I only washed the letter when they were on the bottom and just pushed the cat back and forth to get them on the bottom....I didn't want to drip and on the beauty rings or painted wheels. Right after I washed the letter and rinsed them well. Took about an hour and they turned out pretty white.
Use the method at your own risk.
Bottom letters cleaned top are still brown.