C.R.S. § 37-96.5-103

(1) Precipitation from a rooftop may be collected if: (a) No more than two rain barrels with a combined storage capacity of one hundred ten gallons or less are utilized; (b) Precipitation is collected from the rooftop of a building that is used primarily as a single-family residence or a multi-family residence with four or fewer units; (c) The collected precipitation is used for outdoor purposes including irrigation of lawns and gardens; and (d) The collected precipitation is used on the residential property on which the precipitation is collected. (2) A person shall not use precipitation collected under this article for drinking water or indoor household purposes. (3) The state engineer may curtail rain barrel usage pursuant to section 37-92-502 (2)(a).