Used when there's 2 different navigable channels available, preferred channel markers are placed at the split of channels to indicate that it splits and identifies which is the preferred channel. Like lateral marks, preferred channel buoys have opposite colours depending on the IALA buoyage system you're in and include both port and starboard buoys, each with their own characteristics.
The colour characteristics include a major colour, either red or green, and then a minor colour as a central horizontal stripe, again either green or red. If you're travelling in the direction of buoyage and intend to take the preferred channel, treat the marker as a lateral marker painted in the major colour. If you intend to take the minor channel, treat the marker as a lateral marker painted in the colour of the horizontal stripe.