California Code, Vehicle Code - VEH § 24953

(a) Any turn signal system used to give a signal of intention to turn right or left shall project a flashing white or amber light visible to the front and a flashing red or amber light visible to the rear.

(b) Side-mounted turn signal lamps projecting a flashing amber light to either side may be used to supplement the front and rear turn signals.  Side-mounted turn signal lamps mounted to the rear of the center of the vehicle may project a flashing red light no part of which shall be visible from the front.

(c) In addition to any required turn signal lamps, any vehicle may be equipped with supplemental rear turn signal lamps mounted to the rear of the rearmost portion of the driver's seat in its rearmost position.

(d) In addition to any required or authorized turn signal lamps, any vehicle may be equipped with supplemental rear turn signal lamps that are mounted on, or are an integral portion of, the outside rearview mirrors, so long as the lamps flash simultaneously with the rear turn signal lamps, the light emitted from the lamps is projected only to the rear of the vehicle and is not visible to the driver under normal operating conditions, except for a visual indicator designed to allow monitoring of lamp operation, and the lamps do not project a glaring light.