Colo. Rev. Stat. § 37-48-146
In order to facilitate the preliminary work, the board of directors may borrow money at a net effective interest rate as determined by the board and, as evidence of the debt so contracted, may issue and sell or may issue to contractors or others negotiable evidences of debt, in this article called "warrants", and may pledge, after it has been levied, the preliminary assessment of not exceeding five mills for the repayment thereof, or may pledge the revenue from any service charge or user fee to be levied by the subdistrict. If any warrant so issued by the board of directors is presented for payment and is not paid for want of funds in the treasury, that fact, with the date of presentation, shall be endorsed on the back of such warrant, which shall thereafter draw interest at the rate specified in the endorsement, not exceeding the net effective interest rate as when issued, until such time as there is money on hand sufficient to pay the amount of said warrant with interest.
C.R.S. § 37-48-146