In the old days we washed the car and then used the same soapy water and a brush to clean the whitewalls. They came out nice and bright. Today's whitewalls seem to turn beige after a few weeks and the auto parts stores' shelves are filled with special sprays and nostrums to clean whitewalls. Many of these products work, but leave a cloudy film on the body of the tire or cause it to lose its soft sheen.
The reason whitewalls become brown is because of the synthetic compounds in the tire rubber. As the tire sits these compounds react with air and form the brown film. It cleans off the tire but stains the whitewall.
One of the best — and least expensive — methods of cleaning whitewalls is a bleaching cleanser, such as Comet or Ajax. Wet the tire, cover a wet scrub brush with the cleanser and scrub the whitewall. It will brighten up instantly. Scrub the entire circumference, then wash off the whole tire with soap and water.
If you like to use preserving or shining agents (ArmorAll, etc.) be sure to read the label to see if it should be spread on whitewalls.