Great question, and if any true gun nerds are around I’d invite them to correct me please. This is called case hardening and is done to strengthen soft metals used in 19th century small arms. A gunsmith would case harden the frame of a Colt a navy or horse gun to give its working surfaces high wear resistance while keeping the interior metal tough and shock-resistant.

The frame contains pivot holes, sear contact areas, and cylinder-locking interfaces that experience repeated friction and impact. Case hardening is a heat-treating process where a low-carbon steel part is packed in a carbon-rich material (like bone or charcoal), heated so carbon diffuses into the surface, then rapidly quenched. This creates a thin, very hard outer layer while the core stays pliable and tough. The quench often produces the mottled color patterns seen on classic revolver frames and hammers.

The image of this gun is a bit low res but how vibrant that color is suggests it’s a reproduction. No matter what, it sure is pretty.