The sidelights are coloured either red or green. The sidelights determine the priority on the water: the color displayed on the starboard side is green and the red light is on the port side. The sidelights are located in front of the boat. They each displaying a constant beam and affixed in a manner that the light is projected to the front and sides allow you to determine if the oncoming craft is heading towards you or from one side or another.
They are used to give a person a visual picture of which side of the vessel they are looking at in darkness or reduced visibility. Depending on the situation they will determine who has the right of way in a meeting or crossing situation. If you see a red running light at night crossing or meeting you most likely are the give-way vessel and do not have the right of way. If you see green you most likely are the stand-on vessel and have the right of way.