I have two horns with copper bells -- one a Calicchio flugelhorn with a one-piece copper bell and the other a trumpet with a seamless, plated copper bell. Both horns were unfinished when I bought them. Both bells turned blackish-brown with tarnish -- neither turned green. I had the flugelhorn lacquered. The trumpet is still raw and the bell is dark brown, though a few sections are developing a shiny bluish look. I'm trying to hold off the impulse to polish to see if the blue turns green. My first flugelhorn was a silver-plated Bach. I repeatedly let it tarnish until it was utterly black. After a few years I would have an attack of conscience and polish it up. Then it would slowly turn black again. Of course the silver was gone at the contact points after a few cycles. I never noticed any change in tone or response after a polishing. Of course, I didn't expect to.