I do a lot of lens repairs. Over the last month in particular I've spent most of my time repairing lenses (and not taking photos). The lenses I repair are mostly vintage lenses in varying states of disrepair. I do it partly for fun, partly to save money on buying lenses and mostly to resell and make money. One of the many different tasks involved in repairing lenses is repainting the markings. Old lenses have the different markings for distance, aperture, model, etc. cut into their metal or plastic body which are then fill with paint. Over time these markings can become discoloured as the paint decays/gets dirty. Sometimes the paint just falls off completely. Fortunately because the marking are cut into the lens (unlike modern lenses where they are printed on) the markings can be repaired.
Here I have one of my favourite lenses, the Pentacon 50mm f/1.8. This lenses is in really great condition with the exception that some of the white paint on the aperture markings has become discoloured. If you take a look at the f-stop numbers above you'll notice that the 16 is whiter than the other numbers. That is because I'd already repainted it. This illustrates the yellow discolouration of the old paint.
To demonstrate how to properly repaint lens markings take notice of the 11. First, we need to clean out the old paint with a craft knife. Sometimes simply scraping off the top layer of paint will expose clean paint underneath and you won't need to repaint at all. In this case the paint simply fell out with little encouragement from the knife, so that option was ruled out.
The rest of the process is deceptively simple. Take a paint brush, put a small amount of model paint (whatever colour) and place it on top of the marking. I've heard any cheap acrylic would work just as well. It just so happened that I found a bunch of old model paint that I used in this instance. Then take a cotton swab or a tissue and swab off the paint in a direction that is least likely to smear paint on the lens. Some paint will probably be smeared so use a combination of finger scrubbing and isopropyl alcohol to clean that up. I do need to warn you that there is a danger with smearing paint around and/or stripping the paint off around it. If you keeping scrubbing an area with isopropyl alochol for instance you will strip off the black paint on the lens. Use sparingly. The marking trench should be left with paint inside it, looking good as new.
A final thought here is that with technique can be used for more than just lens restoration. You can do some creative things like repaint your lens silver or gold. I've tried some silver paint myself and it works well. I will likely try a custom lens paint job in the future and post about it here.