Case Coloring is an advanced procedure. Originally, long ago, the colors on the surface of metal were a side-effect of the Case Hardening.
This type of hardening was applied to low carbon steels in order to saturate the surface (1-2 microns) with carbon and make it hard, while the
core stays of mild hardness. That prevents cracks, especially in shotgun receivers and other loaded parts of the gun.
This hardening procedure requires heating parts in container, filled with bone-crash, leather, charcoal to about 1500F,  
and in several hours quenching parts into the water bath.  The colors, obtained as a side-effect are not really stable and wear off quickly,
though hardness remains forever.
In modern circumstances, during restoration of the old guns, the hardness is present but the colors are gone.
So, for restoring colors the original procedure with heating parts can be repeated. However, just for colors it is an overkill.
So, for a number of years we were looking for a less dramatic way of restoring colors on receivers and other parts.
As of now we can create practically any combinations of colors with chemicals, avoiding hi-temp heating.
Note that this is not a painting but chemical reaction on the surface of metal, which is no less stable than the other one.