Bifurcation buoys are placed at the junction of two channels to indicate the preferred (main) channel when a channel splits. When you are travelling upstream, the colour of the top band tells which channel is the preferred channel. You may pass on either side of these buoys.
Starboard Bifurcation Buoys: If red is on the top of a pillar, conical, or spar buoy, the preferred channel is to the left (port) and these buoys are on your starboard (right) side.
Port Bifurcation Buoys; If green is on the top of a pillar, can, or spar buoy, the preferred channel is to the right (starboard) and these buoys are on your port (left) side.