From Auto Geek:
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.
Thus before applying a dressing you really need to do a thorough job of cleaning the outer surface of the tire to remove any spent antiozonants as well as any previously applied dressing and this will prepare the rubber to accept a fresh application of tire dressing.
Meguiar's Tire Dressings:
To maintain your tires and keep them looking sharp with a deep, dark black color, Meguiar's offers six unique tire dressings that treat the rubber with Meguiar's own special conditioning agents as well as patented antiozonants that replenish your tires' original antiozonants and keep your tires looking blacker longer than generic tire dressings and protectants.
Special polymers actually penetrate and cure to the surface to form a long lasting barrier-film that protects your tires from inclement weather and exposure to corrosive elements.
I have been using Meguiar's Tire Foam for years, my tires have worn out before I have seen any cracking.
