Tires turn brown because of the ingredients in the rubber.
Good tires apparently have "Antiozonant" in there to preserve them.
This antiozonant, when exposed to the air, turns brown. They are calling it "Blooming".
It appears we are now stuck with this; and I have read that the best way to prevent it is to seal the tires somehow.
Do a google search: Why are my tires brown?"
https://www.kenwoodtire.com/Blog/art...hile%20driving.