We had an unusual problem with a set of new whitewall tires. Once the tires were mounted we cleaned off the blue soapy coating that's always applied to them for protection. Underneath the coating we found that the whitewalls weren't very bright white, but rather a dingy yellow-brown color as if they were dirty.
This started a cleaning process that frustrated us for quite a while. First we scrubbed the whitewalls with Dobie pads and spray cleaner and got nowhere. There was no improvement, so next we got out the SOS pads and scrubbed away. That too yielded no improvement, so we started to assume that the discoloration was all the way through the whitewall.
To address this situation we tried cleaning areas of one whitewall with solvents. First we tried PrepSol and then lacquer thinner. We also tried to scrub the areas with steel wool and solvent, once again to no avail. We were running out of ideas and resigning ourselves to having dingy whitewalls and decided to clean up all the stuff and go away.
Just before leaving we thought that maybe it would be worth sanding the outer layer of white to see if the color was penetrating all the way into the rubber. We then tested a small area by sanding with 150-grit wet/dry paper and, much to our surprise, a rubbery brown material started sanding away, revealing a much whiter layer. We sanded some more and found that the whitewall was, well, white!
We got a bucket of water and more sandpaper and set out to sand all four tires. One hour later we had sanded off the dingy layer and had four sparkling white tires! What the material was and why it wouldn't come off with steel wool or solvents is beyond us, but now the tires look great and clean up with the more conventional methods.