Color-filled lettering looks amazing on most guns.
Many people try to replicate that color-fill look using nail polish and some even cruder methods. Reddit user ThisIsWhyICarry decided to color-fill his M&P Shield, and the quality of his work if something you would expect to see from the factory. He used flat enamel paint, the kind commonly used on models, to fill in the lettering. The enamel paint provides increased durability over nail polish and it looks great.
If you are looking to color-fill your own gun then we would recommend using this method.