Colo. Rev. Stat. § 37-43-148
At the time of the issue by exchange or sale of refunding bonds authorized under the provisions of sections 37-43-144 to 37-43-151, each bond shall be registered by the district treasurer in a book to be kept by the district treasurer for this purpose. Coupons evidencing unearned interest must be detached and canceled. Each registered bond must have endorsed on the bond the treasurer's certificates of the registration; and only bonds certified in this manner are valid; and the certificate is conclusive evidence that the bond has been duly issued in full conformity with the provisions of sections 37-43-144 to 37-43-151 . Immediately upon the registration of any refunding bond, the treasurer shall certify the fact to the board of county commissioners of the county in which the office of the district is located, in order that the requisite tax levies may be made in due course to meet the maturing interest upon and principal of the bond.
C.R.S. § 37-43-148