Colo. Rev. Stat. § 18-1-705
A person in possession or control of any building, realty, or other premises, or a person who is licensed or privileged to be thereon, is justified in using reasonable and appropriate physical force upon another person when and to the extent that it is reasonably necessary to prevent or terminate what he reasonably believes to be the commission or attempted commission of an unlawful trespass by the other person in or upon the building, realty, or premises. However, he may use deadly force only in defense of himself or another as described in section 18-1-704, or when he reasonably believes it necessary to prevent what he reasonably believes to be an attempt by the trespasser to commit first degree arson.
C.R.S. § 18-1-705
For first degree arson, see § 18-4-102.