Colo. Rev. Stat. § 37-45-138
To pay for construction, operation, and maintenance of said works, and expenses preliminary and incidental thereto, the board is authorized to enter into a contract with the United States or any agency thereof, providing for payment in installments or to issue negotiable bonds of the district. If bonds are authorized, the board shall set a maximum net effective interest rate, and such bonds shall bear interest at a rate such that the net effective interest rate of the issue of bonds does not exceed the maximum net effective interest rate authorized. Interest shall be payable annually or semiannually and shall be due and payable not more than fifty years from their dates. The form, terms, and provisions of said bonds, provisions for their payment, and conditions for their retirement and calling, not inconsistent with law, shall be determined by the board, and they shall be issued in payment of the works, equipment, expenses, and interest during the period of construction. When any bonded indebtedness has been authorized pursuant to section 37-45-139, and when the board has entered into a contract with the United States or any agency thereof whereby the United States or any agency thereof has agreed to purchase such bonds, at an interest rate established in such contract, the board may issue interim notes bearing interest at a net effective interest rate not exceeding the maximum net effective interest rate authorized for the bonds, the interim notes to be payable at the termination of such contract, or at such time not exceeding three years after the date of their issuance and on such terms and conditions as the board may determine. No interim note may be extended or funded except by the issuance of bonds. Bonds also may be issued to pay interim notes as they become due or may be exchanged for the interim notes as the board may determine. Said bonds or interim notes shall be executed in the name of and on behalf of the district and signed by the president of the board with the seal of the district affixed thereto and attested by the secretary of the board. Said bonds or interim notes shall be in such denominations as the board shall determine and shall be payable to bearer and may be registered in the office of the county treasurer of the county wherein the organization of the district has been effected, with the interest coupons payable to bearer, which coupons shall bear the facsimile signature of the president of the board. Such bonds and interim notes shall be exempt from all state, county, municipal, school, and other taxes imposed by any taxing authority of the state of Colorado. Such bonds or interim notes may be sold at par, above par, or below par, but the net effective interest rate to the district, including any discount, but exclusive of any discount payable for costs of the issue, shall not exceed the maximum net effective interest rate authorized for such issue of bonds. Such bonds or interim notes may be used as security for any depository bond or obligation where any kind of bonds or other securities must or may, by law, be deposited as security. Any resolution authorizing, or other instrument appertaining to, any bonds or interim notes under this article may provide that each bond or interim note therein authorized shall recite that it is issued under authority of this article. Such recital shall be conclusive evidence of full compliance with all of the provisions of this article, and all bonds and interim notes issued containing such recital shall be incontestable for any cause whatsoever after their delivery for value.
C.R.S. § 37-45-138