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From the Bureau of Engraving and Printing website : (http://www.moneyfactory.com/document.cfm/18/108)

Quote:

The reason for the selection of green as the color for the backs of currency notes has long been among the more popular questions put to the Bureau. No definite explanation can be made for the original choice; however, it is known that at the time of the introduction of small-sized notes in 1929, the use of green was continued because pigment of that color was readily available in large quantities, the color was relatively high in its resistance to chemical and physical changes, and green was psychologically identified with the strong and stable credit of the Government. In the course of preparing this history, much attention was given to the matter. Extensive research developed evidence in support of the following explanation:

With the growing popularity of banknotes and the development of photography in the mid-1800's, it was customary to print the bills in black combined with colored tints as a deterrent to counterfeiting. The early camera saw everything in black. Features that were distinguishable on a note by color variant lost their individuality when reproduced photographically. However, the counterfeiter soon discovered that the colored inks then in use could easily be removed from a note without disturbing the black ink. He could eradicate the colored portion, photograph the remainder, and then make a desired number of copies to be overprinted with an imitation of the colored parts. The solution to the problem lay in the development of an ink that could not be erased without adversely affecting the black coloring. Such an ink was developed and the patent rights were purchased by Tracy R. Edson, who later was one of the founders of the American Bank Note Company. This is one of the same firms that produced the first paper money issued by the United States. The faces of these and other early notes produced under contract were printed with a green tint, presumably of the protective ink.

It is not unusual in printing with oil-base-type inks, such as was the "patent green," for the color to strike through to the opposite side of a sheet. It might, therefore, be conjectured that the backs of the early notes were printed in a darker shade of ordinary green to make the tint "strike through" less obvious

Since the transition of printing money exclusively at the Treasury Department's Bureau of Engraving and Printing was gradual, it is logical to assume that the backs of the notes produced there during the intervening period were printed in green for the sake of uniformity. Once the Bureau was on full-scale production, there was no reason to change the traditional color and so the practice was continued.