Here is what I do for whitewalls. You will need a circle template with various sizes on it and a white Sharpie marking pen. Find the size you want and place that circle on the tire and fill in whitewall with the white Sharpie. The Sharpie dries a bit flat similar to whiteout and looks fairly good I thought. No sprays and little mess!
Here they are on a Gremlin:
Ski