The old Schwinn whitewall tires would get yellow or discolored over time. They would clean up with some bleach (diluted) and a stiff brush. The later Miyata tires had a different texture to them however, and bleach didn't have much effect. On those we used Armor-All Cleaner (not the protectant you usually see). I don't know if that's still on the market. Formula 409 also worked, but not as well. Keep the brush handy...
How cool or uncool a whitewall is has much to do with what the specific tire looks like, the unicycle it's on, and what you do with it. They're great in shows...
-- johnfoss - More Moab Fun
John Foss, the Uni-Cyclone "jfoss" at "unicycling.com" -- www.unicycling.com