Colo. Rev. Stat. § 43-2-209
If any board of county commissioners desires to let out any work on the county highways by contract, it may advertise in a legal newspaper in the county or post a notice in the county courthouse, for a period of not less than ten days before the contract is let, for sealed proposals for performing the work. When a contract for work on highways involves expenditures equal to or greater than the amount at which a contract requires a contractor's bond under section 38-26-105, C.R.S., the board of county commissioners shall advertise in a newspaper as provided in this section unless such advertisement, in the judgment of the board, would be detrimental to the immediate preservation of the public peace, health, and safety. The advertisement must describe the work to be done and its location and must refer all persons to the person holding the plans and specifications therefor. and such contract shall be awarded to the lowest responsible bidder, the board reserving the right to reject any bids proffered. The cost of any county highway work mentioned in sections 43-2-208 to 43-2-210 may be paid out of the county road and bridge fund or emergency road fund, as the board may determine.
C.R.S. § 43-2-209