Colo. Rev. Stat. § 13-71-142
In all civil and criminal trials, the court may call and impanel alternate jurors to replace jurors who are disqualified or who the court may determine are unable to perform their duties prior to deliberation. Alternate jurors shall be summoned in the same manner, have the same qualifications, be subject to the same examination and challenges, take the same oath, and have the same functions, powers, and privileges as regular jurors. An alternate juror who does not replace a regular juror shall be discharged at the time the jury retires to consider its verdict, unless otherwise provided by law, by agreement of the parties, or by order of the court. The seating of an alternate juror entitles each party to an additional peremptory challenge, which may be exercised as to any prospective jurors.
C.R.S. § 13-71-142