Why do NFL quarterbacks have green circles on their helmets?
According to the NFL: That is to make it easy for the game officials to identify the quarterbacks with a coach-QB transmitter in the helmet. Only one is allowed on the field at at time. As we see more versatile athletes, like the Jets’ Brad Smith, who played quarterback in college and who could line up behind center on occasion, the NFL wants to ensure no team has a competitive advantage. Only one of those helmets with a sticker — for the quarterback — is allowed on the field on any given play.