I've concluded that the people at the U.S. Mint in charge of currency design are incompetent. The $1 coin is the best illustration of this. Sometime in the 1970s, a $1 dollar coin was released with Susan B. Anthony on the front. Unfortunatly, the coin was almost the exact same size and color as a quarter (25 cent piece), and was only produced in small numbers along side the usuall massive production run of $1 bills. It rapidly faded into oblivion.
Two years ago they decided to try again. This time the coin (now featuring Sacajawea, the American Indian guide to Lewis and Clark) was made substantially bigger than the quarter, and a shiny gold color (very similar to the Canadian $1 coin.) They ran a multi-million dollar ad campaign on national TV and distributed the coin in large numbers at major department stores. Then, apparently, they forgot about it, and I haven't seen one in over a year while crisp new $1 bills are still all over the place. They don't seem to grasp that for the coin to be successful, they have to get rid of the bill.
As for colored currency, that's about as likely to show up here anytime soon as the metric system (though metric units are very slowly creeping up on us, especially in pharmaceutical products and beverages.)