The white letters are actually different rubber that's 'painted' white and most tires are 'painted' for a more uniform black.
The 'paint' put on rubber is a special mix of paint, rubber, and a solvent which allows it to bond and become part of the sidewall. It's cured on as well, so I don't know what kind of result you're going to get with white paint. If it doesn't stick it will be easy to get off with an sos pad or even fairly fine steel wool.
And the sos pad is the best thing for cleaning white letters, and for removing the blue that protects them when they're new.
07 Red Ranger Sport 4door, 4 liter, 5 speed, 4x4
265/75/16 BFGoodrich Mud Terrains, Trifecta Tonneau, Silverstars