You're right, Ms Azizi, the USA is not a boring country in the least. :-)

The main reason most countries print different denominations of money in different colours is:

1. It makes it easier to immediately see what denomination the bills are. In Canada, for instance, Ones used to be green, Twos were sort of tan/brown, Fives were and still are bright blue, tens are subdued blue-violet, twentys are pale green and gold, fifties are red, hundreds are sort of gold-coloured, and so on.

2. Harder to counterfeit.

(The Ones and Twos have been recently changed from paper bills to coins. It's cheaper to mint, because the coins last a lot longer in circulation.)