Answer
According to the Federal Motor Carrier Safety Administration General Requirements and Information Marking of CMVs 390.19, the marking must appear on both sides of the CMV (you need at least a pair of magnets), the color of letters must sharply contrast the color of the background on which the letters are placed (black text on white background), and the information must be readily-legible, during daylight hours, from a distance of 50 feet while the CMV is stationary.
We always recommend checking with your local or state DOT for requirements in your area.