When encountering another vessel in darkness or reduced visibility what do visible red and green lights mean?

In darkness or reduced visibility, visible red and green lights on a vessel indicate its orientation and direction of travel. A steady red light on the port (left) side and a steady green light on the starboard (right) side signify that the vessel is underway and facing the observer. If you see both red and green lights, it means you are approaching the bow of the vessel. If only one light is visible, it indicates the vessel's side, helping you assess its direction and take appropriate navigational actions.