Many countries these days are starting to use multicoloured currency, but before that, and in many places still, money is a greenish-grey. Why this colour? Was the easiest to make, or the hardest to discourage counterfeiters? It seems logical that for something to replace precious metals, a gold, silver, or bronze colour would have been chosen. Instead the colour green is pretty much synonymous with money today.
EDIT: I guess I was referring more to the dull greenish-grayish tinge that used to be common on money, while newer bills tend to be brighter/more vibrant.
It's green in America. Is it green anywhere else?
Mod note: let's all pretend that OP asked "Why is American money green?" and answer that question. Please no more pointing out that the rest of the world uses many different colours.
"Greenbacks" were paper money printed during the American Civil War. Before that, the US government never printed paper money, but only issued gold and silver coins. Thus it was the first example of fiat currency in American history.
It was green because, according to the U.S. Bureau of Printing and Engraving, green ink was both plentiful and highly resistant to physical and chemical changes.