Yes I have seen cars with other than brake fluid in the system. The only thing green other than antifreeze that I can think of is two cycle mixing oil. Why people do these things with cars is beyond me.
This is all kind of irrelevant though. If changing your own brake fluid out periodically avoids even the required purchase price of one replacement caliper it is time and money well spent in my opinion. The general dirt in there from disolved or worn rubber parts can accumulate to become a real mess. Then there is the destructive moisture accumulation that can do real damage with time.