I am not familiar with the regulation.Does the color matter, Orange or yellow? Does the passenger need one also?
At least on Navy bases, both operator & passenger require "brightly colored vests" color does not not matter.
Entire instruction can be viewed
here
Quoted text below is a small part from the motorcycle section.
b. Motorcycle Equipment
(1) Motorcycle and ATV operators shall ensure headlights are turned on at all times while the motorcycle is being operated on base except where prohibited by military mission or Status of Forces Agreements.
(2) Government or private motorcycles shall be equipped with a rear view mirror mounted on each side of the handlebars or the left and right side of the fairing.
(3) The following personal protective equipment (PPE) is mandatory for all persons described in paragraph 5a of this instruction
while operating or riding as a passenger on a motorcycle:
(a) A properly fastened (under the chin) protective helmet certified to meet U.S. Department of Transportation (DOT) standards. If the host nation does not have an equivalent helmet standard, the helmet will meet the U.S. DOT standard. Fake or novelty helmets are prohibited.
( B) Properly worn eye protective devices (impact or shatter resistant goggles or full-face shield properly attached to the helmet). A windshield, fairing or eyeglasses alone are not proper eye protection.
© Properly worn long-sleeved shirt or jacket, long- legged trousers and full-fingered gloves or mittens designed for use on a motorcycle.
(d) Sturdy footwear is mandatory. Leather boots or over the ankle shoes are strongly encouraged.
(e)
A brightly colored outer upper garment during the day and a reflective upper garment during the night. The outer upper garment shall be clearly visible and not covered. Military uniforms do not meet these criteria.