Colo. Const. art. X, § 11

Maximum rate of taxation. The rate of taxation on property, for state purposes, shall never exceed four mills on each dollar of valuation; provided, however, that in the discretion of the general assembly an additional levy of not to exceed one mill on each dollar of valuation may from time to time be authorized for the erection of additional buildings at, and for the use, benefit, maintenance, and support of the state educational institutions; provided, further, that the rate of taxation on property for all state purposes, including the additional levy herein provided for, shall never exceed five mills on each dollar of valuation, unless otherwise provided in the constitution.