Why is paper money colored?

The people who issue paper money want to make sure that the money cannot be counterfeited (copied) so they use very elaborate printing schemes to make it difficult to copy paper money. They use colour as just one one of the many protection methods you will find on the money. Other methods are using special paper, invisible inks that can only be seen in ultraviolet light, using embossing, watermarked paper and more recently holographic metal threads woven into the paper.