Colorado Revised Statutes Title 31. Government Municipal § 31-10-1102. Right to vote may be challenged

(1) When any person whose name appears on the registration list or in the registration book makes application for a ballot, his right to vote at that poll and election may be challenged. If the person so applying is not entitled to vote, no ballot shall be delivered to him. Any person may also be challenged when he offers his ballot for deposit in the ballot box.

(2) It is the duty of any judge of election to challenge any person offering to vote who he believes is not a registered elector. In addition, challenges may be made by watchers or any registered elector of the precinct who is present.