Why do we have a red & a green, different flash lights on two wings of a plane?
So you can tell which way another plane is flying at night. If you see both green and red lights, the plane is flying towards you. If you see only green, the plane is passing in front of you from your left to your right. If you see only red, the plane is passing in front of you from your right to your left. If you see red and white, the tail lights, the plane is flying away from you.
