I suppose the mallard is the most familiar duck in the northern hemisphere. The grey body, brilliant green head, yellow beak, and substantial size are the marks of the mallard drake.
About that green head. It isn’t really green at all—that is, there is no green pigment in the feathers. If you took those feathers and crushed them all up, no green would show at all, just a dull brown.
The green is caused by reflection of light from a very, very thin coating on each feather.