One of my hobbies in the 1980s and 1990s was detailing of anything from cameras to bicycle parts, anywhere there was relief lettering on a background.
1.
Prep. With a Q-tip, dip in soapy water and thoroughly scrub as much of the dirt out as you can. A toothbrush can also be used. Dry thoroughly with a lint-free cloth.
2. With some white model-making paint (Humbrol, for example) matched to the colour of the lettering to be restored, dip and thin out a fine sable-hair brush and wipe slowly in a straight line right over the area where the lettering is to be restored — DO NOT use a lot of paint: essentially, it must be enough only to fill/cover the lettering to be restored.
3. Next, tear off a triangular corner of plain white bond paper. Smear your finger in turps and then moisten the surface of the paper in a circular motion. Now whisk steadily that moistened paper in a straight straight over the painted lettering area. Repeat with another unused section of the paper. A finger smeared very, very lightly with turps will also work well but don't press too hard. As you continue you will see over-paint progressively being removed, leaving only paint in that area where it is engraved (or 'set'). Some practice is likely to be needed with this part.
4. Continue to 'fine tune' the finish until all over-paint has been removed.
Allow it all to dry. If all goes well, the paint will settle as it dries. Any extraneous overpaint, likely to be extremely fine smears, can be carefully removed with a Q-tip and metho
when completely dry (don't use turps for this, as any spill into the in-fill areas will ruin your good work!).
Same process with areas that use more than one colour. Do one at a time.
Can you see where I have applied the foregoing on my Manfrotto and GITZO tripods? You can't? Well, must be pretty darned good, hmm?