Colo. Rev. Stat. § 43-2-116
In the event that any fund becomes available from the federal government for expenditure in conjunction with county funds, for the construction, alteration, repair, or improvement of any roads in any county, the board of county commissioners of the respective counties, upon approval by the department of transportation, may use such funds which have accrued to their respective counties from the highway users tax fund for the purpose of matching the federal funds becoming available if the board of county commissioners of any such county has, by proper resolution filed in duplicate with the department of transportation and approved by said department, determined the road construction, alteration, repair, or improvement to be performed in such county and the same is found to conform in all respects to the requirements necessary for the use of such funds of the federal government and if all such funds so available for matching purposes are expended only as provided by law. Any county using highway users tax funds for the purpose of matching federal funds shall be required to reimburse the department of transportation for engineering services rendered by said department in connection with the expenditure of federal funds.
C.R.S. § 43-2-116