What color of light must be displayed by a commercial vessel that is towing at night?

yellow

A common sight on many rivers and waterways are "tugs and tows". When a tug tows or pulls a barge they must display certain lights that identify what they are, and how they are proceeding. Knowing the difference is very important! A tug towing a barge at night might have several hundred feet of cable between the two vessels-cable that is impossible to see. If you don't know what the lights mean, you might think that they are two different vessels and attempt to pass between them at your peril. a tug less than 50 meters in length, with a length of tow under 200 meters. If the tug had the barge alongside, it would show two yellow stern lights, instead of a yellow over white stern light combination.

ETA: Its more than just commercial vessels, that light applies to all vessels that have the ability to tow. Also its not just the color of the tow light, but also the rest of the lights that are required at night. Stern light, side lights, masthead lights (except above the Huey P. Long bridge.)