Case colored gun in the photo is a 1946 vintage Darne R10. I got it in the 1990s, and it had some of the protective lacquer over the colors that the factory applied back then (they didn't do that on guns built in the 1960s, but, I don't know for sure when the practice ended) still on it. I soaked the remains of the lacquer off in dip type carb solvent, and coated the gun with Brownell's Baking Lacquer, following the instructions on the can.
I've used the gun hard. The baking lacquer finish is all still there, and has protected the colors nicely. You need an airbrush to apply baking lacquer, but, it's worth it.
Best,
Ted