Gunkote, BBQ paint, home bluing even.
You could smear it with your own feces...... or whatever
Seriously, there are several ways to go about it. I am thinking of trying bluing on one and have been told that after heat treating you should sandblast because it will get rid of the discolored areas before buing. My MAK90 was blued originally, but I'm thinking of parking and if I do I will send it to Bobby at Salbo arms as he does art! I need at least one gun that has had the spa treatment.