California Code, Vehicle Code - VEH § 40800

(a) A traffic officer on duty for the exclusive or main purpose of enforcing the provisions of Division 10 (commencing with Section 20000) or Division 11 (commencing with Section 21000) shall wear a full distinctive uniform, and if the officer while on duty uses a motor vehicle, it shall be a distinctive color specified by the commissioner.

(b) This section does not apply to an officer assigned exclusively to the duty of investigating and securing evidence in reference to the theft of a vehicle, failure of a person to stop in the event of an accident, violation of Section 23109 or 23109.1, in reference to a felony charge, or to an officer engaged in serving a warrant when the officer is not engaged in patrolling the highways for the purpose of enforcing the traffic laws.