I like this method over enamel or nail polish because it's fool-proof.
If you get sloppy, no problem - just heat it up and wipe it clean.
If you want to sell the gun - just heat and wipe.
If you want a new color - heat and wipe and re-apply.
Step 1: Required materials
Crayon(s) - you can experiment with different colors, I chose white and grey
Stiff paper (I like playing cards)
Notebook or newspaper