Colo. Rev. Stat. § 23-71-605
Such refunding bonds may be exchanged for the bonds to be refunded, or they may be sold at, above, or below their par value; but such refunding bonds shall be exchanged or sold at a price such that the net interest cost and the net effective interest rate for the issue of refunding bonds does not exceed the net interest cost and the net effective interest rate of the outstanding bonds to be refunded or the maximum net effective interest rate and net interest cost approved by the voters, as the case may be. Such refunding bonds shall be in a principal amount not exceeding the principal amount of the bonds to be refunded, as directed by the board of trustees, and the proceeds thereof shall be applied only to the purpose for which such refunding bonds were issued. The principal amount of said refunding bonds may be the same as or less than the principal amount of the bonds to be refunded, if due, adequate, and sufficient provision has been made for the payment or redemption and retirement of said bonds to be refunded and the payment of the interest accruing thereon in accordance with this part 6.
C.R.S. § 23-71-605