I found this info on why tires turn brown along with the white letters and what to do about it interesting. Maybe you will too.
Browning of White Sidewall Letters. BFGoodrich® tire designs include protective agents to retard the natural degradation of the rubber over time, which can occur when exposed to the sun and to ozone present in the air. These protective agentsare designed to slowly migrate to the surface over time in order to provide continual protection over the intended lifespan of the tire. Under normal circumstances the protectants are not noticeable on black rubber or on whiterubber. However, under certain circumstances, these protective agents may become noticeable over time as yellowing or browning due to interaction with ozone. The contrast may be most notable on white rubber. Discolorationof the white rubber is not a warrantable condition. At the same time, exposure to sunlight has a beneficial effect on the appearance of the white rubber. The micro-thin outer surface of the white rubber is designed to photo-oxidizewhen exposed to sunlight. Rain, car washing and even the flexing of a rolling tire will progressively remove these oxidized particles from the white rubber. As particles of the micro-surface layer are removed, new white rubberis revealed. The white rubber of tires on vehicles that are garaged or protected from regular sunlight exposure are not allowed to oxidize or to refresh their surface appearance as designed. As a result, the migration of protectantscan become visible over time on tires not exposed to sunlight Tire sidewalls or letters which have become dull due to dirt or similar buildup can be cleaned with mild soap and water. Should discoloration from migration ofprotective agents occur, regular exposure to sunlight will refresh the white rubber. If regular exposure to sunlight isn’t possible or practical, the following hand cleaning methods could be employed:
Cleaning Option 1: Use common steel wool soap pads (such as Brillo or S.O.S. brands) with water on the white rubber to gentlyremove surface discoloration until the white color is restored. Be sure to rinse thoroughly to remove any remaining cleaning particles.
Cleaning Option 2: Use wet sanding paper (400 grit) with water and mild soap on the white rubber to gently remove surfacediscoloration until the white color is restored. Be sure to rinse thoroughly to remove any remaining cleaning particles. BFGoodrich® Tires does not recommend any powered cleaning methods, either by mechanical, hydraulic,pneumatic, etc. For additional information, please contact your local BFGoodrich® sales representative or contact BFGoodrich® using the website at www.bfgoodrichtires.com
Browning of White Sidewall Letters. BFGoodrich® tire designs include protective agents to retard the natural degradation of the rubber over time, which can occur when exposed to the sun and to ozone present in the air. These protective agentsare designed to slowly migrate to the surface over time in order to provide continual protection over the intended lifespan of the tire. Under normal circumstances the protectants are not noticeable on black rubber or on whiterubber. However, under certain circumstances, these protective agents may become noticeable over time as yellowing or browning due to interaction with ozone. The contrast may be most notable on white rubber. Discolorationof the white rubber is not a warrantable condition. At the same time, exposure to sunlight has a beneficial effect on the appearance of the white rubber. The micro-thin outer surface of the white rubber is designed to photo-oxidizewhen exposed to sunlight. Rain, car washing and even the flexing of a rolling tire will progressively remove these oxidized particles from the white rubber. As particles of the micro-surface layer are removed, new white rubberis revealed. The white rubber of tires on vehicles that are garaged or protected from regular sunlight exposure are not allowed to oxidize or to refresh their surface appearance as designed. As a result, the migration of protectantscan become visible over time on tires not exposed to sunlight Tire sidewalls or letters which have become dull due to dirt or similar buildup can be cleaned with mild soap and water. Should discoloration from migration ofprotective agents occur, regular exposure to sunlight will refresh the white rubber. If regular exposure to sunlight isn’t possible or practical, the following hand cleaning methods could be employed:
Cleaning Option 1: Use common steel wool soap pads (such as Brillo or S.O.S. brands) with water on the white rubber to gentlyremove surface discoloration until the white color is restored. Be sure to rinse thoroughly to remove any remaining cleaning particles.
Cleaning Option 2: Use wet sanding paper (400 grit) with water and mild soap on the white rubber to gently remove surfacediscoloration until the white color is restored. Be sure to rinse thoroughly to remove any remaining cleaning particles. BFGoodrich® Tires does not recommend any powered cleaning methods, either by mechanical, hydraulic,pneumatic, etc. For additional information, please contact your local BFGoodrich® sales representative or contact BFGoodrich® using the website at www.bfgoodrichtires.com