All quarterbacks on each team are required to have 2 helmets on the sideline.
The helmets with the green dot on them have the wireless radio in them so the play can be called into them.
The helmets without the green dot on them have no wireless radio in them.
Green Dot helmets are to be used when the quarterback is on Offense.
The Non-Green Dot helmet is for when the quarterback is holding the ball for PAT's and xtra points.
It would appear that in the past couple of years teams have been cheating on Special Teams and relaying to the quarterback pre-snap the defensive formation. They would then tell the quarterback at the last second if they were going to kick the ball or fake it. The NFL saw this as a violatio of the meaning of the rule. Hence the Green Dots.