Helicopters, like airplanes, have a red and green position light on their sides. The reason for this is the applicable right of way rule (or rather “pre-flight rule”) in air traffic. Just like in German traffic, right before left applies up there and worldwide. The position lights help to recognize in which direction an aircraft is flying in the dark. The green position light is on the front right in the direction of flight and the red position light is on the front left in the direction of flight. In addition, there is a white position light at the rear of the helicopter. In this way, a pilot can see from which direction a helicopter within sight is coming and whether he has the right of way or not. Does he see e.g. B. the red light, the helicopter comes from the right and has the right of way. So our pilot has to let the helicopter coming from the right pass. If our pilot sees the green position light, he has the right of way and may continue to fly. If he sees the green and the red position light, he sees an oncoming helicopter from the front and can react evasively. The right-of-way rule presupposes, of course, that both are approximately at the same altitude. You can find out more about this here.