People v. Reed
In People v. Reed, 932 P.2d 842, 844 (Colo. App. 1996), the Court explained:
"An affirmative defense is a defense that admits the doing of the act charged but seeks to justify, excuse, or mitigate it. . . .
If the prosecution or the defendant presents any credible evidence that an affirmative defense might apply, the prosecution has the burden of proving the non-existence of that affirmative defense beyond a reasonable doubt. Section 18-1-407, C.R.S. . Thus, an affirmative defense is distinguished from an element of the offense because it must be disproved by the prosecution only if the evidence gives rise to an issue with respect to its existence.. . . .
When an exception is included in a statutory section defining the elements of the offense, it is generally the burden of the prosecution to prove that the exception does not apply. However, when an exception is found in a separate clause or is clearly disconnected from the definition of the offense, it is the defendant's burden to claim it as an affirmative defense. . . ."