My enameling friends often use use the “aging” [oxidation] of copper
as an additional color to the enamels. The copper goes through
different colors as it heats. It goes from shiny pink through the
standard copper colors [new penny to old penny] then browns, reds,
black. Experiment with a torch or kiln for the color you want. The
problem is that copper continually oxidizes and if you like the color
you must halt it in some way. In enameling a clear flux is used.
Possibly you could use a laquer or whatever is compatable with your
final goal.
Hope this helps…K. Maxwell