Why modern tires turn brown from the Meguiar's FAQ website:
meguiars.com: FAQ - Which Meguiar's products should I use on my tires?

"The science behind the modern rubber formulas used by major tire manufactures today is both complex and interesting. The rubber itself contains and ingredient called Antiozonant. Antiozonant is an ingredient that helps to prevent the exterior rubber surface from cracking, checking, oxidizing, and deteriorating. The rubber is designed in such a way as to constantly work its way to the outside of the tire and as such, continually replenish the exterior surface with fresh antiozonant.
After the antiozonant works its way to the outside of the tire and is exposed to the ozone in the air, it turns brown. The technical term for this effect is blooming.
This is why you see a brown film on the surface your tires. You can wash your tires with soap or an all-purpose cleaner and remove this film, but in a few weeks, it's back. That's because the antiozonant continually works its way to the outside of the tires every time you drive your car."