What is the difference between verdigris and patina?

i think they're pretty interchangeable, but if you want to be nitpicky:

verdigris is the green coating that accumulates on copper and brass metals.

patina is the green coating that collects on bronze metal. (although patina is used much more broadly than that as well)

taken from definitions below (mac dictionary widget):

verdigris |ˈvərdəˌgrēs; -ˌgris; -ˌgrē|
noun
a bright bluish-green encrustation or patina formed on copper or brass by atmospheric oxidation, consisting of basic copper carbonate.
ORIGIN Middle English : from Old French verte-gres, earlier vert de Grece 'green of Greece.'

patina |pəˈtēnə|
noun
a green or brown film on the surface of bronze or similar metals, produced by oxidation over a long period.