This is an interesting topic. I don't think there's a definitive answer to the question of what weathered copper should look like.
The earlier comment about the copper only turning green when it's out of the water was probably correct. In theory, the surface of the copper gets eroded gradually, constantly exposing a fresh, shiny surface. A couple of other points are relevant, though. First - the copper wasn't necessarily bright and shiny in the first place. Photos seem to suggest that the individual sheets applied to a given ship varied quite a bit in color. Second - copper sheathing prevents some destructive effects of the saltwater environment, but not all. It's great for stopping the boring of the toredo worm, but various forms of marine growth do stick to it. Contemporary accounts tell about streamers of seaweed and lord knows what else clinging to ships' hulls. Interesting modeling challenge - but to my mind not an appealing one.
I've generally handled copper sheathing on plastic models in about the manner that's been discussed - with a mixture of greens, browns, and greys in fine, dry-brushed vertical streaks. The technique of making the nail heads black is interesting, but I'd suggest thinking about it and experimenting with it. Remember that the nails used to fasten copper plates to a wooden hull are copper. (If iron nails are used for that purpose, the result is galvanic corrosion - and the copper falls off.)
One other point - though not relevant to a model of the Victory. Recent research has established that in the latter days of sailing ships copper ("red metal") got replaced by brass ("yellow metal"), or something called "Muntz metal" that apparently was similar in color. Irrelevant for the Constitution or the Victory, but worth thinking about if you're building a Cutty Sark or Flying Cloud.
Hope this helps. Fascinating stuff.
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