Colo. Rev. Stat. § 29-3-107
The principal of, the interest on, and any prior redemption premiums due in connection with the bonds shall be payable from, secured by a pledge of, and constitute a lien on the revenues out of which such bonds shall be made payable. In addition, they may be secured by a mortgage covering all or any part of the project or upon any other property of the user, or both, by a pledge of the revenues from or a financing agreement for such project, or both, as the governing body in its discretion may determine, but no county or municipality shall be authorized hereby to pledge any of its property or to otherwise secure the payment of any bonds with its property; except that the county or municipality may pledge the property of the project or revenues therefrom.
C.R.S. § 29-3-107