Juan,
To get the result that you're after you may want to consider taking away from the tire rather than painting over the white. The raised letters on most tires are a thin rubberized layer that will come off with careful abrasive work. If you have seen cars that run up agaist the curb or trucks off-roading, they tend to scrape off the white lettering.

I have the Goodyear Eagle II's and plan to take off the "Eagle II" to make the tire look more like the billboard racing tires. On the old Dunlops that came with the Spence car I ran a test section on one of the letters and took the white off easily using an air grinder and a flat abrasive pad. Tire black will restore the finish of the existing rubber. It's a mess, but permanent.