Bumping this thread because it came up in another thread and I wanted to add an update.
After two and a half years, the paint is still holding up. None of the gun solvents I use have damaged the paint at all. I had to clean the paint out of Spasmo's G26 when I sold it and it came right out with acrylic thinner. It has to be acrylic thinner, though as regular thinner or mineral spirits wouldn't even touch it.
If you choose to color your engravings, acrylic enamel model paint seems to be the cheapest, easiest, fastest way to go while still giving you not only long-term durability but the ability to change your mind down the road.