As you say, all old copper roof get a green patina (copper oxides and sulphides) , that protects it from corrosion. Only very new roofs look metallic.
Here's some inspiration and tips for all who don't know what we're talking about.
You can protect it from it by covering it with chemicals, but that is a modern thing. It was, and is, beautiful.
The statue of liberty is made of copper.
If you want it realistic it's light green with a hint of blue.
I'd go for VMC Green Sky, Lime Green, and in particular Pastel Green. There's a hint of blue, in particular the shadows. I've also used artist's acrylics blueish phtalo green mixed with white to good effect.

