Jim, it is if in the process you feel your life is in danger. Ronin13 also included that one critical part: "c) you feel your life is in danger".
C.R.S. Section 18-1-704(2)(a) and (b) state that: "Deadly physical force may be used only if a person reasonably believes a lesser degree of force is inadequate and....the actor has reasonable ground to believe, and does believe, that he or another person is in imminent danger of being killed or of receiving great bodily injury, or....the other person is using or reasonably appears about to use physical force against an occupant of a dwelling or business establishment while committing or attempting to commit burglary as defined in sections 18-4-202 to 18-4-204...."
To everyone, it really is important, as Jim said, to read the Colorado Revised Statutes (C.R.S.), which is available in every public library. For anyone who has never done this, it may seem a bit overwhelming at first, but it's really not too difficult to unravel the criminal codes. Just be sure to keep reading because once you think you have your answer, the next paragraph probably provides a "however"! C.R.S. 18-1-704(3), the paragraph following what I quoted above, goes on to say who is NOT JUSTIFIED in using deadly force in those instances above, in subparagraphs (a)(b) and (c). There are always exceptions!