Commonly seen.... and the terminology is confusing... a sailing boat has a *steaming* light for motoring... but it's technical word is masthead. And on motor boats it IS at the head of their mast! My panel uses the word MASTHEAD.. and the NAVIGATION switch turns on bow and stern lights... separate switch for ANCHOR and TRICOLOR lights.OK, sorry. I meant tricolor mounted on the top of the mast. It is an under sail only light.
When motoring, it is the deck mounted nav lights and the white light halfway up the mast, which you are referring to as the masthead light, while I always think of it the steaming light (on our boat, I have the deck mounted nav lights and the masthead light on the same circuit labeled "steaming lights").
The tricolor is not used for motoring, although I am often guilty of forgetting to turn it off when turning on the steaming lights as I drop sails just outside an anchorage to enter at night.
This bit of understandable forgetfulness at a period of much activity transitioning between sail and power, resulting in a boat coming into an anchorage with both its tricolor and deck mounted nav lights on, is why I ignore complaints about it from others already at anchor.