I use Lacquer Stik (fill in paint) quite for awhile. Bought these HERE few years back. It comes in Red, Gold, Black & White about $5 each..
1. burn the tip before rubbing it (circular motion) when the tip gets hard & dry. This helps to fill in the letters nicely.
2. I use Gun Scrubber (birchwood casey) in paper towel to scrub off excess color on the slide.
3. store it in ziplock or any sealed container when not in use to prevent from drying
Definitely go for a crayon. What I do is first wipe down the area that you want to color with alcohol so that no oils from your hands creates a barrier between the wax of the crayon and the slide. Then, just go wild. Use a good bit of pressure so that you fill in what you need to. Then, once you're done, scrape off the excess with a credit card or DL or anything of the sort. Use the alcohol to remove the excess that the card didnt get. Then repeat the process as needed. It took me two coats.
This is the lettering I've done on a pair of my firearms using white fingernail polish. Clean the surface, apply to the stamps or engraving gently and smoothly. Allow to dry. Use a towel with non aceton polish remover and using two or three strong strokes remove the excess.
Saiga 20. This job is actually several years old and has not been touched up. The logo or whatever russian klingon that is on the receiver is not faded or worn, I just didn't bother to 100% it. The old phrase of "it looks better in person" applies here too. The warm color lights muddles things in photos.
When 'illuminating the trades' I had first used the crayon method. While it looked great at first it did not stand the test of time. The crayon melted in the heat of summer in my cars trunk during range trips and it was soft enough either from firing and summer heat that it began picking up grime and coupled with heat and usage just didn't do that well. After the crayon had melted somewhat at times during cleaning it smeared leaving a thin sheen of the wax which caused me to avoid those areas during cleaning. So when I learned about the polish method I gave that a shot and I find that, IME to be the sturdier method.
Hi-Point 9mm Carbine. This is only a few weeks old. Worked well on the High Temp paint job I used on the receiver shroud.