Being from the Chicago area, I believe the above response in correct. There are so many theories as to why the green light and specific to the officer side. Many Illinois fire departments adopted this as well over the years. As an apparatus buff myself, I have noticed many departments that fell into the "blue light craze" on apparatus are now returning to the red/green combination. There are enough people who will recite NFPA blah blah blah......doesn't acknowledge the use of green lights. While it is not an approved color to them, there are ways to properly display the green light in full compliance (even from the showroom).
A few years back, i was on vacation in Florida which always requires from fire apparatus photography. I stopped by a small volunteer department in Lake County which not only had the black over red colors w/ a white stripe, but the green light on the officer side and Chicago style lettering on the doors. I was quite happy to see a local tradition making it down there.