Or some baritone aria, perhaps verde.

A green baritone aria? Do you mean that it is ecologically friendly? Or that it's Irish -- perhaps something from "O'Ida" or "Johnson's Motor Car Men" or "The Barber of Coot Hill"? If the sheet music for the aria was printed on green paper it could be hard to read, much less sing, but it could be done. Or do you mean that the music that it sung is itself green -- and if so, how do you tell what color it is?