You're required to file if you have "taxable Colorado source income," including:
"Income from the ownership of any interest in real or tangible personal property in Colorado. This includes
rents and royalties from real or tangible personal property located in Colorado and the gain or loss from the
sale or other disposition of such property." [CO DOR, Income 6]
See this link: https://www.colorado.gov/pacific/sites/default/files/Income6.pdf.
