In my City of Milwaukee neighborhood, I've noticed that Department of Public Works crews have been out painting fire hydrants. Some of them have been getting green tops.
So, I went to Facebook, posted a photo and tagged some friends who are or have been firefighters, and I got some responses (along with the usual slate of joke comments).
Some friends knew the answer, but, the official word from the city's Water Works Water Distribution Manager Dave Goldapp is that the hydrants with the green bonnets help firefighters responding to an emergency quickly identify a hydrant connected to a water main that is 12 inches or larger.
These are hydrants with, "the capability to support and maintain additional hookups to meet fire suppression needs. The larger water mains provide higher flow and volume than smaller mains."