Yeah, I know NIMMS says that, SFFD actually tried to put green beacons on the roof of the command vehicles but that's another story. SFFD SOPs always put the Command Post IFO the incident building and because we don't want a Chief's buggy in the middle of the fire ground, it (the command post) is rather dynamic and may move as the wind blows. Chief's buggies and the Rescue Squad always park away from the incident, so as not to block any apparatus.
So just to clarify, the green light on the Aerial Trucks, is just to help Apparatus Operators, readily identify what type of vehicle is approaching (or behind) them. The second (or supply engine) would let the Truck go first, so they don't get delayed or blocked. The third engine, can easily see that a second supply is in progress, and would then go around the block, if possible, to supply from the other side.
Hey, it's San Francisco, we do everything different!