I copied this from another site I frequent. My RWL stay white in storage but in the summer months the whole tire seems to turn brown oilyish .Maybe because I do a lot of driving and frequent track blasts. FWIW
The phenomenon effects all modern tires, blackwall, whitewall, and RWL tires.
It is called "tire blooming".
It occurs because modern tire rubber is formulated with anti-ozonants. The purpose of these additives is to prevent premature drying and oxidation that can shorten the life of the tire.
The tire is designed so that these anti-ozonants will be "pushed" over time to the surface of the tire where it makes the surface look more brown than black. It does the same to the white stripe or white letters.
All tires will exhibit this tendency. There can be variation from tire to tire so one set may look worse than another of the same tire, but all will have it. The design of the tire will also play a role in how fast the anti-ozonants migrate to the surface so different tires will exhibit tire blooming more rapidly than others.
As the anti-ozonant additive migrates to the surface, it reacts with oxygen (oxidizes) and creates a brown appearance. You can aggressively remove the material at the surface with a stiff bristle brush and all purpose degreasing cleaners. Personally, I suspect the stiff bristle brush does all the work since you are removing the surface of the rubber to remove the "dead" material, you aren't "cleaning" dirt or grease although the cleaner can help clean surface dirt at the same time.
Regular cleaning will prevent build-up of the oxidized anti-ozonants on the sidewall surface. Lack of regular cleaning will make it more difficult to scrub the surface build-up off when you do try to clean the sidewall.
Once clean, a tire dressing will help slow the reappearance. SIlicone based tire dressings are NOT the cause of tire blooming but can make it more difficult to scrub away the tire blooming. For this reason, water based tire dressings are suggested. They will help to slow the reappearance of the underlying brown discoloration and it will be easier to scrub away when the brown does reappear compared to the silicone based dressings.
Mold release compounds also don't cause the problem but may chemically react with the anti-ozonants to make it more difficult to scrub off the brown. But even if the mold release compounds were thoroughly removed, the browning will still happen.
In the old days, you cleaned road grime off whitewalls and white letters and they came up bright white. I typically used a little Comet cleanser with bleach, left it on for a few minutes after scrubbing. For me, I wasn't done washing my car until those white letters (RWL or the Outline White Letters on the Goodyear GT Radials I had in the late '70s) were blindingly white in the sun.
Those days are gone. The anti-ozonant additive is throughout the rubber in the tire. Exposed to air, it will oxidize and appear brown. And the tire is designed for these anti-ozonants to continually migrate to the surface so the browning will always come back.
Before I understood this, I hated the brown look on my daily driver blackwall tires. I thought it was brake dust or road grime. I would scrub and scrub. But they always looked more murky brown than black.
My '64 GTO came with whitewalls (about half of all '64s had the no cost whitewalls in lieu of the iconic Red Lines). Modern whitewalls in a 14" tire are virtually nonexistent. I didn't want to pay repro Red Line tire prices so I opted for a Uniroyal Tiger Paw AWP II whitewall. Very disappointed that the whitewall looks generally tan with blotchy dark spots to boot.
Now I know why. I can "exfoliate" the surface with a very stiff bristle brush but I know they will never look as bright white as whitewall tires did in the '60s.
Anybody that tells you that old school spray it on and watch the grime run off will work doesn't understand new tire and rubber technology. Black Magic (formerly Westley's) Bleche-Wite is a good bleaching product but it won't remove or whiten oxidized anti-ozonants on the surface of a modern tire. I believe negative reviews of Bleche-Wite are related to the introduction of anti-ozonants to the rubber, not because Bleche-Wite was made inferior when Black Magic took over the brand.
It simply isn't the right product for the job of removing oxidized anti-ozonants.
Like the rest of you, I was hoping for a magic bullet. Don't know what it would take for a tire company to make small batch rubber without anti-ozonant additive but seems to me that is the only possible way for us to get tires that won't turn brown.