OK, that bottom light then should be a combination tail/license plate light. I think the bulb would be a single filament 1156 bulb too. If so, the green wire would be correct for it. You would see a similar setup on the other side, but the light lens would not have a clear bottom.
What color wires do you have on the other side (right side?) You should definitely have green and white, but there should be a third.
If the other bulb in the holder shown has only one filament too, then it is a dedicated stop/turn light which is only on when the brake is applied or you are signaling a turn in that direction.
I'm puzzled by the black wire though. If it is the stop/turn for the other side, then it should not be connected to the red wires at all. Is there possibly a disconnected wire inside the trailer opening sort of hidden?
Also, what year is your HiLo?