USCG rule 25 depending on the length of your vessel you can use a tricolor at the top of the mast for running while you are under sail, if you use the engine under sail you must display a steaming light or another name is mast head light. The mast head light is displayed 225degrees in the front of the mast near the midsection of the mast. The
anchor light is displayed 360 degrees at the top of the mast for when you are at
anchor. For a better picture page 454 in the West Marine catalog will show this.