When on a seafaring vessel, the maritime convention dictates that you stop calling the directions "left" and "right," and that you call them port and starboard instead.

When airplanes became common, and electric lights became the norm for planes and boats alike, a standard for red and green lights was developed to help others determine the vessel's relative heading.

Here's a good way to remember whether red lights go on the left or port side, or not. Unfortunately, like most mnemonics, it's not easy for internationalization.

Thus, the red light does go on the left, er, port side of your plane or vessel, and the green light does belong on the starboard or right side.