Colo. Rev. Stat. § 11-56-110
Refunding bonds, other than special obligation refunding bonds, may be made payable from, and the governing body may pledge to the payment of such refunding bonds, any tax, or any income or revenue of the public body from any source, or both such taxes and such income or revenue, which could have been legally pledged to and used for the payment of the obligations being refunded at the time such obligations being refunded were issued or which could legally be pledged to and used for the payment of the obligations being refunded if the same were to be issued at the time of the refunding, as the governing body may determine, notwithstanding the pledge of such taxes, income or revenue for the payment of the outstanding obligations being refunded is thereby modified. In no event, however, shall any taxes, income, or revenues be pledged for the payment of any refunding bonds which could not have been lawfully pledged for the payment of the obligations being refunded because of the lack of the required approval of such pledge by the electors qualified to vote on the issuance of the obligations being refunded, or because of limitations in the election question authorizing their issuance, unless the refunding bonds and such pledge are similarly authorized by the electors at an election in substantially the manner required by law for the issuance of the obligations being refunded.
C.R.S. § 11-56-110