Colo. Rev. Stat. § 13-90-101
All persons, without exception, other than those specified in sections 13-90-102 to 13-90-108 may be witnesses. Neither parties nor other persons who have an interest in the event of an action or proceeding shall be excluded; nor those who have been convicted of crime; nor persons on account of their opinions on matters of religious belief. In every case the credibility of the witness may be drawn in question, as now provided by law, but the conviction of any person for any felony may be shown for the purpose of affecting the credibility of such witness. The fact of such conviction may be proved like any other fact, not of record, either by the witness himself, who shall be compelled to testify thereto, or by any other person cognizant of such conviction as impeaching testimony or by any other competent testimony. Evidence of a previous conviction of a felony where the witness testifying was convicted five years prior to the time when the witness testifies shall not be admissible in evidence in any civil action.
C.R.S. § 13-90-101