I have always used the red/orange smb for routine and yellow for "trouble" convention, having been trained by British instructors.(if possible I attach a slate/wet note to the yellow smb with a message to the surface cover) I also use the deployment of a yellow smb up the orange smb line to indicate that I require extra deco gas.( I leave a rig on the boat) As there is no universal standard it is important to make sure that the boat/back up team know what any signal means during the dive brief.