You should file a CO nonresident return. The Colorado Individual Income Tax Filing Guide (p.3) states:
Generally, you must file this return if you are required to file a federal income tax return with the IRS for this year or will have a Colorado income tax liability for this year and you are:
• A full-year resident of Colorado, or
• A part-year Colorado resident who received taxable income while residing here (you must file the DR 0104 along with the DR 0104PN), or
• Not a resident of Colorado, but received income from sources within Colorado (you must file the DR 0104 along with the DR 0104PN).
The instructions refer to "income." Rental receipts are considered income.
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