ORIGINAL: touch and go
The rules of the sea, and the air, are that if two vessels, or aircraft, are on a collision course, such as converging at a right angle to each other, the vessel/aircraft on the right has the right of way. Thus, by putting the green light on the right side of a vessel or aircraft and the red light on the left side, as the two converge the captain of the craft on the right side will see a green light on the other craft indicating that he has the right of way and the captain on the left side will see a red light indicating that he does not have the right of way. That's why the lights are set up as they are.
Makes perfect sense. I had always wondered, too, why the colors were the way they were. I remember which is which by the way ship channels are marked, red to the right side on entry to the channel from seaward, or Red Right Return. An aircraft coming toward you (returning) would have its left wing (red light) to YOUR right, so Red Right Return works there also.