I live in the Chicagoland areaand have had a few oppurtunities to visit some of my friends in Chicago. I had one of my daughters with me during one such visit and she asked me, "daddy, why does that fire truck have a green light instead of a red one like your truck?" ...
So, like any good father and made up some lie since fathers are always supposed to have answers for their little girls. Hell, I really don't remember what I told her but I have to say she had planted a question that I needed answered as well. I asked around.

It seems when Chicago FD was getting out of the horse owning business and into the motorized carriage-truck phase alot of options were up in the air. The horse drawn hose wagons and pumpers that had roofs were always dirty and mildewy and wet. They rotted quite easily since they were made of wood. That's when some bright young firefighter got the idea to tar the roof and make it waterproof. The rest of the wagon was red since red was a very durable color and easy to maintain. The coachlights on the officers side (passenger front) were green. Not because there was an EMT on board but because there was a fire commisioner (Goodrich) who had been a sailor and steamshipman. During a particularly rough fire he couldn't tell which end of the wagon/truck was which in some very heavy smoke. He then provided the chauffeur with a nautical green lamp (starboard) and told him to place it onto the passenger front so at a distance or in heavy smoke he might have a chance telling which end was pointed where. Red was used on the (left) port side. He did this for all the new motorized fleet as well. So when ordering a fire truck with the "Chicago Package" the roof of the cab is black with a red body and a green light is on the officer's side of the truck.