There are several different companies which offer metal Patinas, Dyes and Oxides some that have very little to no chemical base .
As I stated earlier , I had at one time experimented with oxides and could not achieve the same results as what I has seen on some of Derringers works .
Let me clarify that by saying , this would be concerning early muzzle loading works where a blue , red and even yellow color was added to a browned or blued finished barrel .Not to be confused with a heavy Fuax Damascus pattern often seen on shotguns and some pistols of the mid 19th and early 20th century
Now most certainly this could be do to the process being different then what Derringer used . However they do make it possible to apply copper and bronze onto a metal surface without electroplating or the uses of acids or mercury .
Myself I did not find the patinas to hold up well for firearms applications . Dyes and oxides however did not do to bad . In fact I still have a test barrel which I applied a yellow and blue Dye oxide to produce a pattern steel . that’s been
Maybe 10 years now . The pattern and color is still visible where as the barrel that I applied the patina to has long since sloughed its colors
This is pistol carries a brown and red patina over copper rub . Its pretty hard to see any color in the photo . It is there . . I was able to achieve what I felt was the proper thickness . However I could not produce a quality , sharp pattern nor was there any durability with actual use .
Now on this example I applied Dye-oxide to barrel steel . This was with 2 coats of each . No attempt was made to recreate an accurate pattern. This was simply a test to see how the oxide would build and contrast .. The blue was applied then using a brush I simply randomly removed the blue with solvent . Then applied the yellow .
This was one of the last oxide pattern tests I did . Its also one of the ones that’s held up quite well . It however did not produce the results I was looking for and thus was not repeated .
Anyway , good luck in your search in recreating the derringer patterns.