California Code, Elections Code - ELEC § 13230

(a) If the county elections official determines that, due to the number of candidates and measures that must be printed on the ballot, the ballot will be larger than may be conveniently handled, the county elections official may provide that a nonpartisan ballot shall be given to each partisan voter, together with his or her partisan ballot, and that the material appearing under the heading “Voter-Nominated and Nonpartisan Offices” on partisan ballots, as well as the heading itself, shall be omitted from the partisan ballots.

(b) If the county elections official so provides, the procedure prescribed for the handling and canvassing of ballots shall be modified to the extent necessary to permit the use of two ballots by partisan voters. The county elections official may, in this case, order the second ballot to be printed on paper of a different tint, and assign to those ballots numbers higher than those assigned to the ballots containing partisan offices.

(c) “Partisan voters,” for purposes of this section, includes both persons who have disclosed a party preference pursuant to Section 2151 or 2152 and persons who have declined to disclose a party preference, but who have chosen to vote the ballot of a political party as authorized by that party's rules duly noticed to the Secretary of State.