Colo. Rev. Stat. § 39-8-101
The board of county commissioners of each county of the state, except the city and county of Denver and the city and county of Broomfield, shall comprise the board of equalization of such county. In the city and county of Denver, the board of equalization shall be comprised of such of its officers as may be provided by its charter. In the city and county of Broomfield, the board of equalization shall be the city council or a board or commission appointed by the city council. A majority of the board shall constitute a quorum, and no official action shall be taken at any meeting of the board unless a quorum is present.
C.R.S. § 39-8-101
For constitutional grant of powers to county boards of equalization, see § 15 of article X of the state constitution.