Colo. Rev. Stat. § 13-64-401
No person shall be qualified to testify as an expert witness concerning issues of negligence in any medical malpractice action or proceeding against a physician unless he not only is a licensed physician but can demonstrate by competent evidence that, as a result of training, education, knowledge, and experience in the evaluation, diagnosis, and treatment of the disease or injury which is the subject matter of the action or proceeding against the physician defendant, he was substantially familiar with applicable standards of care and practice as they relate to the act or omission which is the subject of the claim on the date of the incident. The court shall not permit an expert in one medical subspecialty to testify against a physician in another medical subspecialty unless, in addition to such a showing of substantial familiarity, there is a showing that the standards of care and practice in the two fields are similar. The limitations in this section shall not apply to expert witnesses testifying as to the degree or permanency of medical or physical impairment.
C.R.S. § 13-64-401