C.R.S. § 13-40-108: Service of notice to vacate or demand.

(1) Except as provided in subsection (2) of this section, a written notice to vacate or demand as required by section 13-40-104, 38-12-202, 38-12-204, or 38-12-1303 may be served by delivering a copy of the notice to a known tenant occupying the premises, or by leaving the copy with a person or a member of the tenant's family who is older than fifteen years of age, residing on or in charge of the premises, or, in case no one is on the premises after attempts at personal service at least once on two separate days, by posting the copy in a conspicuous place on the premises.

(2) If a tenant provides the landlord with written or actual notice that the tenant is a victim-survivor of unlawful sexual behavior, stalking, domestic violence, or domestic abuse, the landlord shall make all reasonable efforts to perfect service described in subsection (1) of this section through personal service to the tenant who provided the written or actual notice. If personal service cannot be perfected after three attempts, the landlord shall perfect service by posting a copy of the notice in a conspicuous place on the premises and by serving the notice to the tenant through certified mail or through any commercial mail courier if the notice includes a receipt or other documentation demonstrating proof of delivery.