Copper roofs, exposed to air and rain water, do indeed turn a remarkably bright, bluish green - unless, of course, the copper has had some sort of treatment applied to its surface. Here in Greenville, North Carolina, we have a lovely old Queen Anne-style house, the Humber House, that's owned by the state and recently has been undergoing some restoration - including the installation of a brand new copper (?) roof. It was interesting to watch what happened to it. Initially it was a bright, brilliant copper color. Then (I think) the roofers sprayed some sort of preservative on the surface. Since then, the roof has been a shiny, slightly-metallic-looking, dark brown. I think that's due to whatever coating was applied to the metal. Another old building nearby has a green dome.
What happens to copper submerged in saltwater is another matter.
The problem gets rather complicated when one starts to consider all the variables. For one thing, it seems that only in the early years of metal-sheathed hulls was the metal actually copper. Sometime before the middle of the nineteenth century the metalurgists started trying to develop more durable - and economical - alternatives. Suppliers' advertisements from the 1850s list "red metal" (presumably copper), "yellow metal" (brass), and a couple of others.
Just what happened to a piece of sheathing metal when it was applied to a ship's hull and submerged in salt water for a prolongued period? The theory was that as long as the ship was in motion the surface of the metal would constantly be eroding, and the exposed surface would always be bright copper in color. I have my doubts about that.
Then there's the question of how uniform in color the stuff was in the first place, before it was nailed to the hull. (My guess is: not very.) Some modelers like to prepare the copper they're going to put on their hulls by heating it with a torch, making some plates brown, some slightly greenish, and some almost pure black. Those people cite, as evidence, a number of photos from the nineteenth century that show metal-sheathed hulls in drydock; the plates do indeed seem to show a tremendous variation in color. I have my doubts about that logic, too. Heat and saltwater don't produce the same effects on metal. And it's easy to be deceived in those old photos by the way sunlight bounces off surfaces that are at a slight angle to each other.
It's worth remembering that a genuinely accurate model of a hull that had been in the water for several months probably wouldn't be the sort of thing most of us would want in our living rooms. It would be adorned with seaweed, barnacles, and heaven only knows what other kinds of marine growth. If somebody wants to make some 1/96-scale barnacles and stick them onto his model's hull, great. That somebody will not be me.
There are lots of practical, attractive approaches to this problem. I, for one, have no intention of declaring any of them "right" or "wrong."