The colors of the “bonnet,” or top, denote the amount of water available in the hydrant, measured in a gallons-per-minute flow rate. The National Fire Protection Association recommends a certain color scheme, along with other best practices for testing and marking hydrants.
A blue bonnet means at least 1,500 GPM are available, possibly more. (Some hydrants are designed for 4,000 GPM.) A green bonnet means the flow is between 1,000 and 1,500 GPM.
Most of Denton’s 5,344 fire hydrants are inspected regularly and are designed to meet high standards, said city spokesman Ryan Adams. The city also paints hydrant bodies a silver color to heighten the contrast with the bonnet color.
Other bonnet colors specified by the association include orange (500-999 GPM) and red (less than 500 GPM). In Denton, private fire hydrants are supposed to be marked red, as well.