Colo. Rev. Stat. § 16-10-105
The court may direct that a sufficient number of jurors in addition to the regular jury be called and impaneled to sit as alternate jurors. Alternate jurors in the order in which they are called shall replace jurors who, prior to the time the jury retires to consider its verdict, become unable or disqualified to perform their duties. Alternate jurors shall be drawn in the same manner, shall have the same qualifications, shall be subject to the same examination and challenges, shall take the same oath, and shall have the same functions, powers, facilities, and privileges as the regular jurors. An alternate juror shall be discharged when the jury retires to consider its verdict or at such time as determined by the court. When alternate jurors are impaneled, each side is entitled to one peremptory challenge in addition to those otherwise allowed by law. In a case in which a class 1, 2, or 3 felony, as described in section 18-1.3-401(1)(a)(IV) and (1)(a)(V), C.R.S., is charged, and in a case in which a level 1 or level 2 drug felony as described in section 18-1.3-401.5, C.R.S., is charged, and in any case in which a felony listed in section 24-4.1-302(1), C.R.S., is charged, the court shall impanel at least one juror to sit as an alternate if requested by any party.
C.R.S. § 16-10-105