Colo. Rev. Stat. § 37-26-105
The refunding bonds shall mature serially, the first payment to be in not more than five years and the last payment in not more than thirty-five years from the date of the bonds; but each bond issued shall be redeemable at the option of the district five years prior to its maturity and on any interest-paying date thereafter. Interest on any bond called for payment shall cease thirty days after publication of a notice of call in a newspaper published or of general circulation in the district. Consistently with the denomination of the bonds issued, maturities shall be in substantially equal annual amounts of principal, or in such amounts as will require substantially equal annual assessments for principal and interest throughout the period, commencing not later than five years after the date of the bonds.
C.R.S. § 37-26-105