I must admit I have noticed before that some have green tops, and other colors, but I never knew the significance behind it. I always figured they just ran out of red paint or red tops.
A couple weeks ago my friend was reading a book with her son, and found out the reason behind all the different color tops. I found it very fascinating, and have since then been studying all the fire hydrants as I drive by.
It's even turned into a game with me and my children. We all like to point out the different tops that we see. I have found the most common color to be green, and red to actually be one of the more uncommon.
The lid color is made different to allow firefighters to quickly know how fast the water flows out of the hydrant.
The blue lid signifies the fastest flow rate at 1500 gallons per minute, or higher.
The green lid signifies the next fastest flow rate between 1000 gallons per minute to 1499 gallons per minute.
The orange lid signifies a flow rate between 500 to 999 gallons per minute.
The red lid signifies the slowest flow rate under 500 gallons per minute.
-*Amendment*- I recently spotted a white topped fire hydrant and was especially interested to find out what it stands for.
A white or purple top indicates that the hydrant provides non-potable water. (You can't drink it)