There are a couple companies that make tire stickers made of rubber. They have permanent/semi permanent options. You basically just glue the letters one-by-one to the sidewall. Any type of white lettering on your tires will eventually brown. Using the tire paint pens will brown much faster than using tire stickers. Paint pens may stay white a few days to a couple weeks, depending on how often you drive/how clean the road is you're driving on/how well you prepped the tire before application.
Here's my car at a track day this past spring. I had finished the rear tires the day before, and they had already turned brown from the surface I was driving on. The fronts stood up just fine.
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NASAHPDE0419-209 by
Khoi Ha, on Flickr[/IMG]