C.R.S. § 39-22-603

Wherever in this article it is required that a return be made under oath, the signing of the return by the person therein required to make oath shall be sufficient compliance with the provisions of said sections if such return contains or is verified by a written declaration that it is made under the penalties of perjury in the second degree. Any individual who willfully makes and signs a return which he does not believe to be true and correct as to every material matter is guilty of perjury in the second degree.