Colo. Const. art. VII, § 8

All elections by the people shall be by ballot, and in case paper ballots are required to be used, no ballots shall be marked in any way whereby the ballot can be identified as the ballot of the person casting it. The election officers shall be sworn or affirmed not to inquire or disclose how any elector shall have voted. In all cases of contested election in which paper ballots are required to be used, the ballots cast may be counted and compared with the list of voters, and examined under such safeguards and regulations as may be provided by law. Nothing in this section, however, shall be construed to prevent the use of any machine or mechanical contrivance for the purpose of receiving and registering the votes cast at any election, provided that secrecy in voting is preserved.

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 therefor 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.