Colo. Const. art. X, § 19

State income tax laws by reference to United States tax laws. The general assembly may by law define the income upon which income taxes may be levied under section 17 of this article by reference to provisions of the laws of the United States in effect from time to time, whether retrospective or prospective in their operation, and shall in any such law provide the dollar amount of personal exemptions to be allowed to the taxpayer as a deduction. The general assembly may in any such law provide for other exceptions or modifications to any of such provisions of the laws of the United States and for retrospective exceptions or modifications to those provisions which are retrospective.