Colo. Const. art. II, § 23

Trial by jury - grand jury. The right of trial by jury shall remain inviolate in criminal cases; but a jury in civil cases in all courts, or in criminal cases in courts not of record, may consist of less than twelve persons, as may be prescribed by law. Hereafter a grand jury shall consist of twelve persons, any nine of whom concurring may find an indictment; provided, the general assembly may change, regulate or abolish the grand jury system; and provided, further, the right of any person to serve on any jury shall not be denied or abridged on account of sex, and the general assembly may provide by law for the exemption from jury service of persons or classes of persons.