I came from the Philippines where the bills have different colors for different denominations:
Then the US bills. I find the the US bills are the least colorful currencies. The spectrum of colors is so limited – green or black on white background on ALL the denominations. From 2003, new redesigned bills were introduced, the first significant design change in 67 years. These bills features new and subtle background colors, but they still look too green and too alike!
I have once mistaken a $10 to $1. I can’t blame the money for that. I can only blame myself for not paying attention. The bills just look the same to me. If I have given the $10 bill to a cashier, I would have gotten some change, but I gave the $10 bill as a tip for a service rendered that cost less than $10. And you don’t get change from tips.
Of course, there is reason for US money to be green. The ink used is green. Making bills out of this special green ink makes the bills difficult to counterfeat. The introduction of new redesigned bills with the new and subtle background colors is an attempt to stay ahead of currency counterfeiters.