Don't know if this is the "credit" you're referring to, but here are CO's rules on the Capital Gains Subtraction:
https://www.colorado.gov/pacific/sites/default/files/Income15.pdf
Note that real estate purchased before May 9, 1994, or on or after June 4, 2009, does not qualify.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
