Colo. Rev. Stat. § 37-26-101
The board of directors of any drainage district in this state has the power, under the conditions in this article, to issue negotiable coupon bonds to be denominated "refunding bonds" for the purpose of paying, redeeming, or compromising outstanding bonds of the district and any unpaid matured interest on such outstanding bonds, whether such outstanding bonds are due or not, or payable at the option of the district or by consent of the bondholders, and whether such bonds are outstanding or are issued on or after April 24, 1933. Such refunding bonds shall not exceed in amount the principal sum of the bonds outstanding and the unpaid matured interest thereon at the time of the issue of the refunding bonds. The rate of interest on such refunding bonds shall not exceed the rate on the bonds so refunded, and in no event shall interest exceed six percent per annum. Such refunding bonds may be issued and used to redeem or compromise all or any part of one or more issues of bonds outstanding at the time of refunding, together with unpaid matured interest thereon.
C.R.S. § 37-26-101