Colo. Rev. Stat. § 43-2-117
If any proposed county road is on the county line between two counties, the board of county commissioners of each county interested shall proceed in the same manner provided in section 43-2-112, and the board of county commissioners of each interested county by mutual agreement shall designate the county road supervisor who shall survey the proposed road and make the report to said boards in the same manner as provided in section 43-2-112; and the concurrence of the boards of county commissioners of both counties shall be necessary to establish it. If any such road is established, each of such counties shall open and maintain a definite part thereof, which the board of county commissioners of such counties shall apportion by mutual agreement between the two counties or by application of subsection (4) of section 43-2-111, and if the boards of county commissioners cannot agree upon the apportionment, it may refer the matter to three disinterested freeholders as arbitrators, whose duty it shall be to apportion same and report thereon to the boards of county commissioners of both counties.
C.R.S. § 43-2-117