Colo. Rev. Stat. § 30-35-803
The issuance of any bonds voted on in accordance with this part 8 shall be authorized by an ordinance, which shall be irrepealable until the bonds therein provided for shall have been fully paid or discharged, specifying the purpose to which the funds to be raised shall be applied and providing for the levy of a tax sufficient to pay the annual interest and extinguish the principal of such bonds within fifteen but not less than ten years from the creation thereof. Such tax, when collected, shall be applied only to the purpose specified in such ordinance until such bonds shall be paid or discharged. Such bonds shall bear interest at a rate or rates and shall be exchanged or sold at a price or prices so that the net effective interest rate of the issue of bonds does not exceed the maximum net effective interest rate authorized. Interest shall be paid semiannually and shall be payable at such place, be in such denominations, and be executed by such officers as may be prescribed in such ordinance. Such bonds may be exchanged for outstanding matured and overdue special assessment bonds or obligations and interest thereon or they may be sold and the proceeds thereof used for the purpose specified in this part 8.
C.R.S. § 30-35-803