I believe that some American double gun makers applied some kind of protecting finish over the colors in their guns.
All did. Not just US-Americans. Zapon lacquer, back then.
Carcano
I don't believe Parker coated their color case hardened frames & parts with anything.
Neither did AH Fox at least when they used Bone/Wood charcoal process on the earlier guns.
The cyanide colored guns , maybe something was put on them.
AH Fox didn't do the cyanide hardening in house. They had (Fred) Heinzelman (sp?) in Carlstadt, NJ do that work for them.
They are still in biz AFAIK. They were a couple of yrs ago at least and would still take in polished firearms parts for the Cyanide CAse Color work.
They had (still have?) an FFL so sending the frame was no problem. Cost was cheap as I recall.
That's where Turnbull sent his Fox work and any other that needed cyanide coloring back when.
Don't know what they do know.
Most everything got 'coated' with a protective something there. All the SxS's that I remember except the Parkers.
Orange shellac was a fav.
Yes the color layer is incredibly thin on the surface.
Depending on how the re-coloring was done, the colors would sometimes start to wear off the high edges
just during reassembly. Especially if you had to take-apart & reassemble a couple times to get that GD SST to work right.
Just some idle thoughts this AM along w/a cup of coffee