The COLREGS Rule 27b states "A vessel restricted in her ability to manoeuvre (a vessel which from the nature of her work is restricted in her ability to maneuver ), except a vessel engaged in mine clearance operations, shall exhibit: three all-round lights in a vertical line where they can best be seen. The highest and lowest of these lights shall be red and the middle light shall be white".
Simply put this means a ship displaying RED-WHITE-RED lights is working and everyone else should stay clear.
A statement in any stateroom. These matte, museum-quality posters are printed on durable, archival paper that is suitable for framing.