Colo. Const. art. X, § 18

License fees and excise taxes - use of. On and after July 1, 1935, the proceeds from the imposition of any license, registration fee, or other charge with respect to the operation of any motor vehicle upon any public highway in this state and the proceeds from the imposition of any excise tax on gasoline or other liquid motor fuel except aviation fuel used for aviation purposes shall, except costs of administration, be used exclusively for the construction, maintenance, and supervision of the public highways of this state. Any taxes imposed upon aviation fuel shall be used exclusively for aviation purposes.