Colo. Const. art. II, § 11
No ex post facto law, nor law impairing the obligation of contracts, or retrospective in its operation, or making any irrevocable grant of special privileges, franchises or immunities, shall be passed by the general assembly.
Source: Entire article added, effective August 1, 1876, see L. 1877, p. 30.
Cross references: For retrospective laws, see § 12 of article XV of this constitution.