If you see only a green light or only a red light, what does it signify in relation to another vessel?

When observing a single colored light—either green or red—on another vessel, it signifies your position relative to that vessel and indicates which type of vessel you are encountering. A green light typically indicates the starboard (right) side of a vessel, while a red light signifies the port (left) side.

Seeing only a red light indicates the other vessel is on your port side, and it might be another powered vessel or a sailing vessel under sail. Under normal navigation rules, as a vessel approaching another vessel that presents only one light, you must give way to the vessel showing the red light, which means you must take action to avoid a collision.

Alternatively, if you see only a green light, it means the other vessel is on your starboard side; in this case, you would be the stand-on vessel and maintain your course. Therefore, correctly interpreting these light signals is crucial for safe navigation and understanding right-of-way rules on the water.