Did you know World War II U.S. vehicle registration numbers used to be blue?

Look at almost any photo of a vehicle in U.S. service during World War II, and you’ll see a white registration number painted somewhere – on jeeps, (The Jeep), for example, it’s on the side of the hood. This number was stenciled there by an official government inspector once the vehicle was accepted at the factory. What you might not have known, however, is that the number was actually in blue paint, and it was only painted over in white once the vehicle was received by a unit in the field.

The numbers were originally painted white or yellow, but were changed to blue in 1942. The reason for the blue color was security. Each registration number was unique, and while not every possible number was used, it was still feared that industrial spies or enemy scouts in the field might be able to deduce the total number produced of vehicles of various types by recording enough numbers and figuring out the scheme. The blue drab paint of the number silhouetted against the olive drab paint job was very hard to make out from a distance and didn’t show up well in black-and-white photographs, so the enemy could not record the numbers.

The problem was that the two colors melded into each other too well, and the numbers could be hard to read even up-close, causing all sorts of administrative and logistical problems. Therefore, instructions came from the top that the numbers must be repainted in white (for better contrast) once the vehicles were on the field.