Colo. Rev. Stat. § 32-11-529
(1) The district may issue as special obligations any of the following types of district securities, in anticipation of net pledged revenues; but not under any circumstances under their terms and the proceedings authorizing their issuance, in anticipation of taxes nor in anticipation of gross pledged revenues:
Source: L. 69: p. 772, § 71. C.R.S. 1963: § 89-21-71.