Colo. Rev. Stat. § 37-43-146
The board of directors at the same election, or at any future general or special election, may submit to the qualified voters of the district the question of whether the coupons upon such refunding bonds shall bear interest at a rate not to exceed six percent per annum after their maturity, if said coupons are not paid upon presentation. In such case the ballots shall contain the words, "Interest on coupons - Yes", and "Interest on coupons - No", or words equivalent thereto. The vote upon said question shall be canvassed at the same time and in the same manner as the vote for the issue of refunding bonds and, if a majority of the qualified electors of the district have voted "Interest on coupons - Yes", the board of directors shall certify the result and cause the same to be entered of record and filed the same as in the case of an election for refunding bonds. Thereafter, when any of said coupons are presented for payment on or after their due dates and there are insufficient funds on hand to pay said coupons, the county treasurer shall stamp on the back of said coupons, "Not paid for want of funds", and all coupons so stamped shall bear interest from that date until paid at the same rate as the principal of the bond.
C.R.S. § 37-43-146