IMHO, I think you are going the wrong direction with 'paints' that try to cover up the rubber. Latex will not bond to the rubber and just peel off. I think you will have better luck looking for a stain type product, urethane or lacquer based. How about trying stained glass window stains available at art supply stores, or that transparent yellow (more like orange)lacquer that they sell to paint light bulbs for turn signals? Scrub the letters VERY CLEAN with lacquer thinner B4 staining.
The above assumes you are trying to color existing white lettering. If you are trying to color black lettering, that is tough. Best bet is a white lacquer or white urethane sold by POR15. Clean the rubber VERY WELL with lacquer thinner and scrub the surface with 100 grit sand paper B4 applying the paint. Then stain the newly white letters as above.
[ April 30, 2003: Message edited by: firstname.lastname@example.org ]</p>