Usually three reasons why the tires "brown-out".
1. Brand of tire, some are more prone.
2. Cleaning of the tire with chemicals that are excessively caustic and remove the micro wax and carbon black from the sidewall.
3. The use of the "greasy kid stuff" as I call it.
Give a wet, does not really dry for a few days. This is a dimethal silicone/petroluem distillate dressing (some even have hexane in them, a strong, flammable solvent added to the regular distillate)
This removes the micro wax and carbon black just as caustic waterbased cleaners will. The difference it that the solvent liquifies the wax and carbon black and then it slings and stains the flex agent parts.
Do your sidewall exhibit a "rough to the touch", do they look really flat and have the brown tinge?
Then it is most likely the dressing you or a carwash has been using.
We wrote a few years back a whole tech article on this subject for one of the trade journals. Ford picked up on it and put it on their dealer only website.
If you wish to read about it,
www.autoint.com and go to the Magazine Articles Section.
Anyway, that is what are the usual reasons behind your concern.
