Pickling is still done with sulfuric acid as a prep for nickel plating. Citric acid doesn't seem to react with cuprous (red) copper oxide as sulfuric and nitric acid will, and it takes some more time on cupric (black) copper oxide than it does on verdigris (copper carbonate here, but also copper chloride near salt water). So the pinkish hue left after using citric acid to clean a heavily tarnished case has to be polished off if you want it removed. You can see it in the post-cleaning photo below.
