Colo. Const. art. XI, § 4

Law creating debt. In no case shall any debt above mentioned in this article be created except by a law which shall be irrepealable, until the indebtedness therein provided for shall have been fully paid or discharged; such law shall specify the purposes to which the funds so raised shall be applied, and provide for the levy of a tax sufficient to pay the interest on and extinguish the principal of such debt within the time limited by such law for the payment thereof, which in the case of debts contracted for the erection of public buildings and supplying deficiencies of revenue shall not be less than ten nor more than fifteen years, and the funds arising from the collection of any such tax shall not be applied to any other purpose than that provided in the law levying the same, and when the debt thereby created shall be paid or discharged, such tax shall cease and the balance, if any, to the credit of the fund shall immediately be placed to the credit of the general fund of the state.