Colorado constitution
Article VII
Section 8

When the governing body of any county, city, city and county or town, including the city and county of Denver, and any city, city and county or town which may be governed by the provisions of special charter, shall adopt and purchase a voting machine, or voting machines, such governing body may provide for the payment therefore by the issuance of interest ­bearing bonds, certificates of indebtedness or other obligations, which shall be a charge upon such city, city and county, or town; such bonds, certificates or other obligations may be made payable at such time or times, not exceeding ten years from date of issue, as may be determined, but shall not be issued or sold at less than par.