Colorado Revised statute 42-4-1204 (2) says:
(2) Except as otherwise provided in subsection (4) of this section, in addition to the restrictions specified in subsection (1) of this section, no person shall stand or park a vehicle, except when necessary to avoid conflict with other traffic or in compliance with the directions of a police officer or an official traffic control device, in any of the following places:
(b) Within fifteen feet of a fire hydrant;
So it's a state law, but I suppose local municipalities can make more restrictive rules if they wanted.
Subsection (4) exempts bicycles from the parking rules if you were wondering.