Not much:
http://articles.chicagotribune.com/...ey-uncle-samQuote:
Q-I own a $5 bill that`s blue on the back side, not green. How much might this printing error be worth?
B. M., Chicago
A-Just $5, the bill`s face value.
The green ink on U. S. paper money has yellow and blue components, and tricksters have discovered that certain acids will create a ``blue back`` by destroying the yellow ink. Similarly, some alkalis will produce a ``yellow back`` by destroying the blue ink. Experts say that Uncle Sam has yet to produce a Federal Reserve note with the wrong-color ink