Azizi, everywhere else in the world uses coloured money. FWIW, the general opinion is that US dollars look like cheap toy money. Well, they practically are since the dollar went down the crapper, but let's not get into that now. Using different colours (and changing the designs regularly) makes them harder to fake.

There's also a practical issue - most other countries have notes that are different sizes for different denominations. Not only does this help against fakes, it also allows the blind to use cash. It's an interesting little factoid that blind people in the US can't tell bills apart, because they're all the same size.

Graham.