Shooting technical expert
This is called colour hardening, and it is a traditional way of finishing actions.
In other industrial processes it is known as case hardening, a process which forms a very hard surface on steel.
It works like this: A steel component is heated, and kept hot for some time, while buried in charcoal.
Carbon from the charcoal is absorbed into the surface of the steel, forming a very hard crust.
Gun makers discovered that, by adding other components to the charcoal, colours, primarily blues and browns, could be brought out in an abstract pattern.
The additives to the charcoal were many and varied, and often kept secret from competitors.
One of them was tiny pieces of leather. Case hardening does not always produce a pattern of colours.
For many years the deep blue finish on Colt handguns was much admired.
This was achieved by heating the components in charcoal made from animal bones.