I have done this many times for my vintage builds. It's easier if you are painting larger letters that have a small border of rubber around them rather than paint raised lettering.
For large letters with a rubber border: use polycarbonate paint (white etc). Dip a tiny phillips head screwdriver into the paint and dab it inside the letter. The paint will release from the screwdriver tip and start to fill into the letter [kind of like filling a pool with water]. Do this for each letter.. You can use the edge of an x-acto blade to scrape away any paint that overflowed outside the letter.
For small letters raised from the tire: use the same polycarbonate paint, except use a -drybrush- technique to paint them carefully with a brush. To drybrush; you will want to dip a very small brush into the paint and wipe most of it away on a paper plate etc. Do this until only a tiny bit of paint remains on the brush. Now lightly brush it across the raised letters. The tacky paint remaining on the brush will rub off onto the letters for a clean professional finish... You will need to do this several times to complete all of the lettering, but it will looks really nice when you are done.
Hope this helps.
[Here is a sample of large letters I did using the 'screwdriver' technique]
