Thanks to schulte who posted this in another thread.
As can be found explicitly in Texas general statutes,
(b) Except as expressly authorized by law, a person may not
operate or move equipment or a vehicle, other than a police vehicle,
with a lamp or device that displays a red light visible from
directly in front of the center of the equipment or vehicle.
(c) A person may not operate a motor vehicle equipped with a
red, white, or blue beacon, flashing, or alternating light unless
the equipment is:
(1) used as specifically authorized by this chapter;
(2) a running lamp, headlamp, taillamp, backup lamp,
or turn signal lamp that is used as authorized by law.
And, more specifically:
§ 547.303. COLOR REQUIREMENTS. (a) Unless expressly
provided otherwise, a lighting device or reflector mounted on the
rear of a vehicle must be or reflect red.
(b) A signaling device mounted on the rear of a vehicle may
be red, amber, or yellow.