Colo. Rev. Stat. § 39-11-149
If two or more noncontiguous lots, tracts of land, or mining claims or portions thereof have not been separately valued and assessed or, having been separately valued and assessed, whether having a common ownership or not, have had tax liens thereof sold en masse for a gross sum for the nonpayment of taxes and charges thereon, then, after seven years from the date of any such sale, such assessment and sale and any tax sale certificate issued thereon shall be deemed valid and legal and shall be so considered in all actions, suits, or proceedings in which is involved the validity of any such assessment, sale, tax sale certificate, or treasurer's deed issued thereon. There is excepted from this section any such action, suit, or proceeding pending on August 1, 1964, wherein any party thereto has or may assert the invalidity of any such assessment, sale, tax sale certificate, or treasurer's deed. Nothing in this section shall be construed to alter, amend, or repeal section 39-11-148.
C.R.S. § 39-11-149