Reply #36: The goal is to keep them "green", while still adding color.

[Xithras]

I saw a great photo once that had the currencies of every nation in the world lined up in a huge block. US currency was instantly recognizeable and stuck out like a sore thumb because its green on green design is visually distinct in the sea of currency colors. The "greenback" is instantly recognized anywhere in the world because of its design.

The Mint had a tough job on these bills. They had to add color while STILL keeping the greenback green and the general design recognizeably "American". They had to add color to the bills without undermining the visual distinctiveness that makes our currency unique.