Colo. Rev. Stat. § 39-26-125
The taxes for any period, together with the interest thereon and penalties with respect thereto, imposed by this part 1 shall not be assessed, nor shall any notice of lien be filed, or distraint warrant issued, or suit for collection be instituted, nor any other action to collect the same be commenced, more than three years after the date on which the tax was or is payable, except as set forth in section 29-2-106.1 (5)(b); nor shall any lien continue after such period, except for taxes assessed before the expiration of such period, notice of lien with respect to which has been filed prior to the expiration of such period, in which cases such lien shall continue only for one year after the filing of notice thereof. In the case of a false or fraudulent return with intent to evade tax, the tax, together with interest and penalties thereon, may be assessed, or proceedings for the collection of such taxes, may be begun, at any time. Before the expiration of such period of limitation, the taxpayer and the executive director of the department of revenue may agree in writing to an extension thereof, and the period so agreed on may be extended by subsequent agreements in writing.
C.R.S. § 39-26-125