Colorado constitution
Article XX
Section 5

The city council, or board of trustees, or other body in which the legislative powers of any home rule city or town may then be vested, on its own initiative, may submit any measure, charter amendment, or the question whether or not a charter convention shall be called, at any general or special state or municipal election held not less than 30 days after the effective date of the ordinance or resolution submitting such question to the voters.