I have used the salt and ammonia procedure for getting a blue patina
on copper, and never had a problem with it changing color. It is a
matt finish which is what I wanted.
I also got a permanent verdigris finish quite by accident once, and
have used it ever since. I was doing some lead soldering on a copper
pipe (making a trellis for my garden), and some of the flux used with
the solder got on the copper, and turned this marvelous blue/green
finish. I then took a paint brush, and painted the entire trelliswith
the flux and to my delight it took. That trellis has been in my
garden, exposed to all sorts of weather, rain, snow, etc., and still
has that verdigris finish.
I put some on some copper that I was combining with enamels in a
large wall piece, and again, the color is true and has held up.
You can get the flux from any stained glass place—it is the one
stained glass artists use when soldering.
Having said that, I do think some research may be needed to find out
if it would be harmful to the skin if applied to a piece of copper to
be incorporated into jewelry—even after it has dried fully.