They probably have the same problem that's been the problem with raised white letter tires for the last few years. There are many, many posts about this on the forums.
https://forums.maxperformanceinc.com...+turning+brown
Almost appears like oil stains leeching from the inside out. I gave up on RWL tires because of it.
On the other hand, if you are not having that same problem, there are lots of suggestions posted in the thread above.
Since all this came up a couple of years ago, I've noticed that even the black part of tires are turning brown nowadays if they aren't freshly cleaned and treated with your favorite tire dressing. Tire dressing will mask it on the the black parts but not on the white.