Colo. Rev. Stat. § 31-15-712
All work done by the city in the construction of works of public improvement of five thousand dollars or more shall be done by contract to the lowest responsible bidder on open bids after ample advertisement. It shall be unlawful for any person to divide a works of public improvement construction into two or more separate projects for the sole purpose of evading or attempting to evade the requirement that works of public improvement construction costing five thousand dollars or more be submitted to open bidding, unless the total cost of any such project would be less if divided into two or more projects than if submitted to open bidding as one project. If no bids are received or if, in the opinion of the city council, all bids received are too high, the city may enter into negotiations concerning the contract. No negotiated price shall exceed the lowest responsible bid previously received. The city is not required to advertise for and receive bids for such technical, professional, or incidental assistance as it may deem wise to employ in guarding the interest of the city against the neglect of contractors in the performance of such work.
C.R.S. § 31-15-712
This section is similar to former § 31-15-103 as it existed prior to 1975.