I think you will find that the NHTSB has the final say. Although the following is termed a "recommendation", the NHTSB expects nation wide adherence to its recommendations.
"All school buses should:
Be identified with the words "School Bus" printed in letters not less than eight inches high, located between the warning signal lamps as high as possible without impairing visibility of the lettering from both front and rear, and have no other lettering on the front or rear of the vehicle, except as required by Federal Motor Vehicle Safety Standards (FMVSS), 49 CFR part 571.
Be painted National School Bus Glossy Yellow, in accordance with the colorimetric specification of National Institute of Standards and Technology (NIST) Federal Standard No. 595a, Color 13432, except that the hood should be either that color or lusterless black, matching NIST Federal Standard No. 595a, Color 37038.
Have bumpers of glossy black, matching NIST Federal Standard No. 595a, Color 17038, unless, for increased visibility, they are covered with a reflective material.
Be equipped with safety equipment for use in an emergency, including a charged fire extinguisher, that is properly mounted near the driver's seat, with signs indicating the location of such equipment.
Be equipped with device(s) demonstrated to enhance the safe operation of school vehicles, such as a stop signal arm.
Be equipped with a system of signal lamps that conforms to the school bus requirements of FMVSS No. 108, 49 CFR 571.108.
Have a system of mirrors that conforms to the school bus requirements of FMVSS No. 111, 49 CFR 571.111.
Comply with all FMVSS applicable to school buses at the time of their manufacture.
Any school bus meeting the identification recommendations of sections l, a-h above that is permanently converted for use wholly for purposes other than transporting children to and from school or school-related events should be painted a color other than National School Bus Glossy Yellow, and should have the stop arms and school bus signal lamps described by sections 1, e & f removed."