There are two different marks that are used to mark the edge of a channel - Lateral Marks. They are green and red in colour, and the side of the channel on which each is placed depends on the direction of buoyage and the IALA region in which they are found.

In IALA Region A (the UK and most of the world) when entering a port or harbour, the red (port hand) marks are kept to your left, whilst the green starboard-hand marks are left to the right

Bifurcation Marks

Sometimes a channel splits into two, with navigable water in both channels. If this is the case, a Bifurcation mark may be used, to indicate which is the preferred channel to use.

Bifurcation marks are green and red striped, with the colour of the middle stripe being the one indicating to which side the primary channel lies; if the post is primarily green with a red stripe the the middle then the preferred channel is to port, and vice versa if the post is mainly red with a green middle stripe.