Yellow = Buoy not controlled by any faction.
A buoy, often called a flag, is the flag-like structure found on blockade boards that is central to their scoring mechanism. Each buoy may appear with one to three pennants on it. If at the end of the blockade turn, a buoy is within the influence circle of one or more ships of a single alliance, points are scored for that alliance corresponding to the number of pennants on the buoy. Unclaimed buoys and buoys contended by more than one alliance are not scored for any side.
A buoy may appear in one of six colors. Yellow is unclaimed, red is claimed by the attacking alliance, green is claimed by the defending alliance, blue is claimed by the observer's alliance, grey is claimed by a non-contender, and black is contested by multiple alliances. Note that ships can be on the same square as a buoy, and that going over one does not cause any sort of damage to a ship.