I found some videos on Youtube with instructions on how to add colors to your firearms' engravings. Some use Crayons, some with Modelling Paints, but I chose to do mine using Nail Polish because I felt it would be more durable than using Crayons, and yet easier to find than other paint materials. Here is the video that I used for instructions:
http://www.youtube.com/watch?v=vcLzaxqsOCEThis works not only on the slide, but on the Polymer magazines as well. Here are the end results on my Glock 22:
Just follow the instructions on the video, and you should be fine. A couple of important notes, though to remember, though, based on my experience doing this:
'Choosing the Materials'
I used the "Caronia" brand for the Nail Polish, and "Bobbie" brand for the Nail Polish Remover, although any other brand you can find will do. These can easily be bought at your neighborhood SM Department Store. If the Saleslady starts giving you strange looks, just explain right away that you are using these for your gun's engravings. Of course, she will probably just think, "Yeah, right". The polish and remover are dirt cheap, I was able to buy 2 colors and 1 remover for under P100.
When choosing the Nail Polish colors, remember to choose "bright" colors (i.e. White, Yellow, etc.) as much as possible. If you are to use "dark" colors (i.e., Red, Blue, etc.), try to use the "brighter" versions of these dark colors, like "Bright Red", for example. Or "Neon Blue". The reason is that if you don't use them, you won't be able to see much the colors once it dries against the flat black background of the slide. One suggestion is that if you can't find a bright version of Red, then Orange is a good substitute.