I agree with tokeneer, some of my silver strikes get a toning, that it is very difficult to tell the difference. I am not sure if you dipped a suspected copper, if it was a true copper, that the color would remain the copper color or not.
I know that a regular brass, after dipping would prove it was either a copper or brass. (turn to the brass color)
Or you could do a chemical test like Tokeneer.
(I am not sure how this is done).
Jay