Found it Ed...this is probably the section the officer was referring to. That makes 2 equipment violations the officer could have cited the op's friend for if the officer felt like pushing the issue.

From www.leginfo.ca.gov, Vehicle Code:

25950. This section applies to the color of lamps and to any
reflector exhibiting or reflecting perceptible light of 0.05 candela
or more per foot-candle of incident illumination. Unless provided
otherwise, the color of lamps and reflectors upon a vehicle shall be
as follows:
[SNIP]
(b) The emitted light from all lamps and the reflected light from
all reflectors, visible from the rear of a vehicle, shall be red
except as follows:
(1) Stoplamps on vehicles manufactured before January 1, 1979, may
show yellow to the rear.
(2) Turn signal lamps may show yellow to the rear.
(3) Front side marker lamps required by Section 25100 may show
yellow to the rear.
(4) Backup lamps shall show white to the rear.
(5) The rearward facing portion of a front-mounted double-faced
turn signal lamp may show amber to the rear while the headlamps or
parking lamps are lighted, if the intensity of the light emitted is
not greater than the parking lamps and the turn signal function is
not impaired.
(6) A reflector meeting the requirements of, and installed in
accordance with, Section 24611 shall be red or white, or both.

@StrangeWill: Thanks!