It's true...an acceptable facsimile of a "color case hardened" finish can be made by using cold gun blue and Naval Jelly rust remover.
You remove the hammer or trigger part, of course. Polish bright where you want the cch to go, degrease using a cleaning solvent, and heat it briefly using a hair dryer, a propane torch, or even on top of a stove burner. Nothing extreme, just too hot to hold with your fingers.
Using a cotton tipped swab, dab the surface in an irregular pattern...swirl some, streak and dot others. Let the surface dry, and then repeat. The different applications will give darker blue, gray, and greenish to some areas, and lighter than others. Overlap the patterns to make it look natural. Use the Naval Jelly to lighten or fade some of the darker areas. Rinse with warm soapy water to stop the action.
I did a couple of beatup finish-worn antique Winchesters and Marlins this way and have had comments about how good the case hardened finish was before I told them it was home-brewed.
Oil to seal in and protect the finish. Even better, degrease again, and use clear polyeurethane spray to protect the finish.
