Beginning in the 2007 NFL season, quarterbacks begain wearing a green dot on the back of their helmet. In 2008, defensive players begain wearing the green dot on their football helmets. What's up with the green dot on the player's helmets?
Helmets with green dots have radio receivers in them. Coaches can call in plays from the sidelines. However, only one player is allowed on the field at a time with the green dot helmet. Also, the radio reception only works for a certain amount of time prior to the play starting. This prevents "last-second" changes to the plays being called in from the sidelines.
The bottom line... one offensive and one defensive player can have a green dot helmet on the football field at any given time.
Now you know.