Colo. Rev. Stat. § 30-35-708
Any such refunding bonds may be issued to refund one or more issues of outstanding bonds of a county; but no two or more issues of outstanding bonds may be refunded by a single issue of refunding bonds unless the taxable property upon which tax levies are being made for payment of each such outstanding issue of bonds is identical to the taxable property on which such levies are being made for the payment of all other outstanding bonds proposed to be refunded by such single issue of refunding bonds. In the event that two or more issues of outstanding bonds of a county are to be refunded by the issuance of a single issue of refunding bonds, as provided in this section, the net interest cost and net effective interest rate on the bonds to be refunded shall be computed as if all of said bonds had originally been combined as a single issue aggregating the total of the smaller issues, and the results of this computation shall be compared with the net interest cost and net effective interest rate on the whole of the single refunding issue for purposes of determining the necessity of submitting the question of issuing such refunding bonds to a vote of the registered electors of the county.
C.R.S. § 30-35-708