I used the Testors model paint. I have done 6 guns so far. Here us what I have learned.
1- Clean the area well.
2- Cover the entire lettering area. It will look like a paint stripe on the gun.
3- Use a hair dryer to fully dry the paint. Also helps to completely fill the engraving.
4- Only do one color at a time. If you do 2 at once they have a tendency to bleed into each other. Do one, wait til fully dry and complete, then do other.
5- Once paint is dry use a white lint free cloth to wipe off excess. I have used gun cleaning patches with good success. Make sure the cloth is stretched tight on your finger. Wet it with CLP. Work back and forth over the paint with constant pressure. It will take a while but will start to come off. Rotate to a clean spot on the cloth and apply more CLP frequently. Once it starts coming off it goes pretty quick. Wipe with a clean dry cloth several times throughout the process.
6- Once cleaned up, hit it again with the blow dryer. Helps to really set the paint in the engraving.
Works and looks great. Way more durable than crayons.