Colo. Rev. Stat. § 30-6-109
The territory so stricken off from any county and annexed to the adjoining county shall be held to pay its ratable proportion of all then existing liabilities of the county from which it has been taken. Such ratable proportion of liabilities, as soon as the proclamation has been made, shall be fixed by the board of county commissioners of the county from which it is taken, and certified to the board of county commissioners of the county of which it becomes a part; and said board of county commissioners of the last mentioned county shall cause a special tax to be levied upon the property subject to taxation in such annexed territory for one, two, or three years, until such ratable proportion has been fully collected and paid, and the money, when collected, shall be refunded to the county from which the territory has been taken.
C.R.S. § 30-6-109
For effect on pending actions of the merger of a municipality into the city and county of Denver, see § 30-11-201.