Colo. Rev. Stat. § 31-23-305
Such regulations, restrictions, and boundaries may from time to time be amended, supplemented, changed, modified, or repealed. In case, however, of a protest against changes in regulations or restrictions, or changes in the zone district applicable to particular land, which protest is filed with the municipal clerk at least twenty-four hours prior to the governing body's vote on the change and is signed by the owners of twenty percent or more of the area of land which is subject to the proposed change or twenty percent or more of the area of land extending a radius of one hundred feet from the land which is subject to the proposed change, disregarding intervening public streets and alleys, such changes shall not become effective except by the favorable vote of two-thirds of all the members of the governing body of the municipality. The provisions of section 31-23-304 relative to public hearings and official notice shall apply equally to all changes or amendments.
C.R.S. § 31-23-305
This section is similar to former § 31-23-205 as it existed prior to 1975.