What color for repainting lens engravings (Yashica ML)?

Although I haven't done a partial repaint of lettering on anything where a portion of the affected section is still intact and legible - regardless of current or original coloring - I have done a total repaint of things like f-stop markings, IR index marks and DoF legends, and even logos.. However I should note that my personal preference on much older gear (i.e., 1920-1950's, or older) and anything with a fair bit of rarity - is that I tend to leave those alone, unless it's important to be visible and legible from a usability standpoint. I wouldn't consider that on any ML or similar 'modern' lens or body, though.

I prefer to use paint sticks ('oil' paint, by Uchida or Markel) and totally stay away from liquid paint and brushes altogether. That's due to my advanced years, and not my advanced skills. :) It takes quite a while to dry completely but without being runny while it does, so it can be reworked and pushed into crevices and wiped away on any areas it shouldn't remain on. My dexterity and patience certainly aren't that bad, but I find it too fidgety to try to get a nice controllable flow of liquid paint into a very precise and very small indentation without overfilling  whatever cavity I'm trying to rework and end up with excess and smeared paint, and then have to clean it up and redo it all.

I haven't tried it, but I suppose if you'd prefer to match colors and only do a partial refill, you could probably do some limited premixing with paint sticks by scraping off some of two or more colors and mash-mix them. I use a stainless steel  dental spatula for several tasks, including repaints on lens rings as a filling and occasionally as a scrape-off tool, and I'd assume it would work well as a painter's palette knife. The limitation is in the number of basic colors available in stick form. There's usually only primary colors in a couple of shade variations, plus some browns, grays, and black. Mixing any paint - solid or liquid - is doable with trial and error and letting the liquid types test dry first since they'll change intensity. Sticks shouldn't change color much, if at all.

I have also used enamel paint pens, but only for light touch ups on black, white, or silver body or barrel pieces. It can dry too quickly and be a bit globby, both during and after, and I'd assume there's no way to mix them effectively.

I've also used nail polish for alignment dots (using toothpicks as droppers), since you can get an amazing array of red shades, and if you're doing lens cleanings or repair, it can also be heavily thinned with Naptha or polish remover, and dotted onto inner lens screws to keep them in place.

One thing I would not recommend, is using anything like Naptha or nail polish remover directly on anything as a cleaner or a removing solvent, unless you want to do a complete finish removal. It can also de-texture a smooth finish into the equivalent of 20 grit sandpaper. It will eat paint like it's breakfast food to a starving 2 year old! I'd suggest just 91-99% Isopropyl alcohol as the last cleaner on a cotton swab just before painting, regardless of the paint type you'd choose.

As always, and as my last caveat for later readers... test first, before cleaning/applying anything, so you'll be familiar with how things work, and what never to do or use again.