It depends on the configuration seen. There are different lights visible depending on the relative headings of the vessels and relative sizes etc.
You are approaching the starboard side of another vessel
The other vessel is coming directly toward the observer, so both the port and starboard running lights are visible.
You are approaching a powerboat head-on
The other vessel is coming directly toward the observer, so both the port and starboard running lights are visible.
You are approaching a sailing vessel - head-on
You are approaching a powerboat head-on
You are approaching a powerboat head-on
You are approaching the port side of another vessel
Turn on navigation lights.
Turn on navigation lights.
Turn on navigation lights.
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.