Emergency lights are only allowed on authorized emergency vehicles in California. Volunteers aren't allowed to have any lights and must obey all traffic laws in personal vehicles....
According to the California Vehicle Code:
25251.5. (a) Any motor vehicle may also be equipped with a system
in which an amber light is center mounted on the rear of a vehicle to
communicate a component of deceleration of the vehicle, and which
light pulses in a controlled fashion at a rate which varies
exponentially with a component of deceleration.
(b) Any motor vehicle may be equipped with two amber lamps on the
rear of the vehicle which operate simultaneously with not more than
four flashes within four seconds after the accelerator pedal is in
the deceleration position and which are not lighted at any other
time. The lamps shall be mounted at the same height, with one lamp
located on each side of the vertical centerline of the vehicle, not
higher than the bottom of the rear window, or if the vehicle has no
rear window, not higher than 60 inches. The light output from each of
the lamps shall not exceed 200 candlepower at any angle horizontal
or above. The amber lamps may be used either separately or in
combination with another lamp.
(c) Any stoplamp or supplemental stoplamp required or permitted by
Section 24603 may be equipped so as to flash not more than four
times within the first four seconds after actuation by application of
the brakes.
I'm not suggesting that folks should run out and equip their POV's but the CVC seems to imply it's ok to add amber lights that meet the guidelines.