ive dealt with the white letter issue a few times on BFG tires & the last set they replaced for me were much better but still turn brown after sitting for a couple months. i have found that cleaning them after sitting in direct sunlight for awhile helps a lot, the sun/heat must release the brown crap better & lets wesleys bleech white or other cleaner work much better. read about cleaning after sitting in the sun on another forum where a member was having the same problems.
also found a couple products that are supposed to deal with this issue on RWL or white walls. this one is supposed to fix the leeching of teh UV chemical that is what causes them to turn brown, i spoke to the owner & he guarantees the product will work or money back. i have yet to try it becuuse the cleaning process i mentioned has helped a lot & is acceptable for now but for the price this stuff looks like its worth a try.
http://lasteffortgarage.com/wax.html