"Rich Hampel" wrote
The green color is probably coming from 'verdigris' a copper acetate
compound which forms when copper or copper oxide is in contact with
Acetic Acid. ..... which depending on the acid content (pH) of the
water will turn greenish or brown/purplish.

Interesting. I suspected it might be sort of a patina or something. Main
reason I asked is I want to touch up some thin spots and, having no
experience with this stuff, was concerned I might be putting VC-17 over
something that might not be VC-17.