You're welcome. I'm glad someone else likes them.
Generally speaking, the buoys mark a navigable channel or hazards in or near the channel, like wrecks, reefs, etc. Yellow buoys generally marl anchorages in large harbors, or are NOAA weather buoys. Mooring buoys are generally round (plastic toilet floats) painted white with a blue band.
The USCG does paint and set at least one buoy that has no navigational significance. The Francis Scott Key buoy marks the approximate position of HMS TONNANT in Baltimore Harbor, where Francis Scott Key wrote The Star Spangled Banner.