The KKK-A-1822 standard was/is a standard from the federal government for ambulances, which requires them to be white with an orange stripe. But like any standard (NFPA), it doesn't have to be followed.
http://www.sysign.com/Ref_Files/KKK-A-1822F.pdf
http://www.sysign.com/Ref_Files/KKK-A-1822F.pdf