Yellow buoys are used to caution boats to stay away from them. They can be used to indicate that there are underwater pipes, dredged lines or a shoal area. Yellow is used as a general warning on buoys, but those that are all yellow should be kept away from.
Yellow buoys may well be lighted and will be lighted with yellow lights. There are a wide range of different colored buoys, each with a different meaning. The green and red buoys are used in tandem and are used to mark the side of a channel when sailing to or from a smaller body of water. The greens mark the port side of the channel and the red buoys mark the starboard side when returning. Unlighted green buoys are often called canned cans and unlighted red buoys are called nuns.
A buoy that is both red and green is used to mark a junction, used to indicate a possible obstruction with two ways around it. They are often marked with a preferred and a secondary channel to take. Buoys that are both red and white, colored vertically, mark safe water and indicate that there is a good depth of water around them and are encountered further out than a green or red buoy.