However, this is my AR, and I really like the white and red filled selector markings on HK firearms and wanted to replicate it on my firearm.
There are several ways of doing it, and I used a red lumber crayon on the AR in the video
Basically I just rubbed the crayon over the inscription and then wiped off the excess. Just like ivory scrimshaw, the colorant filled the deeper cuts and was able to be wiped clean of the body.
You can dab enamel paint or fingernail polish into the engraving for a more durable color, but it is harder to remove it from the side of the gun. If you use a rag with too much thinner it will drip in and mix with the paint you want to keep. Not enough and it can smear on the body of your gun.
If you use crayon, be aware heat can make it run
If you use nail polish and decide to remove it, a tooth brush and nail polish remover does a good job.
I like how this looks, but you need to be careful what you use, and ensure its what you really want. Some people like the look that comes from coloring AR-15 receiver markings, but many do not.