Rob , The color code goes like this.......

White = ground
brown = tail lites/marker lites
green = right hand turn signal
yellow= left turn signal

The fifth wire (blue) is typically tied to your back up lights circuit and its purpose is to disable the boat trailer's "surge brakes" when backing up. If its a big boat with electric brakes requiring a "Brake controller" then the blue wire could be used for the output from the controller . You need to first determine if the blue wire is used for trailer braking .

Hope this helps......... Ken