The US Treasury Department, in an effort to thwart counterfeiters, will be introducing $20 bills with a new background color (e. g. something other than green) on March 27th. See: http://money.cnn.com/2003/03/05/news/money/index.htm
According to the article, the bills will have "a predominant but subtle color [in] the background".
Digital counterfeiting has become a serious problem, with 40% of all counterfeiting being done that way in 2002. However, according to an anti-counterfeiting expert cited in the article, the new color probably won't do much to counteract the problem, as many counterfeiters just bleach money and then digitally print the face of a larger bill on top. The way you can tell that such bills are counterfeit is because the security strip and watermark for the smaller bill remain. The only way he could think of to combat this problem would be for the Mint to use some sort of penetrating ink.
So, what color of cash would you like to see? Are we going to start calling twenties mauvebacks or chartreusebacks instead of greenbacks? Will it be the long teal rather than the long green?