Why are us dollar bills green?

The color was developed around the time of the Civil War as an anti-counterfeiting measure. The color was difficult to copy using the relatively primitive photographic and printing techniques then available.

By the 1990s the development of digital copiers and high-quality commercial printers made the color a liability, though. Many other countries had already switched to intricate, multi-colored bills in a race against counterfeiting technology, and after considerable political wrangling the US followed suit by redesigning the $5 and all higher denomination bills. The Treasury decided that the risk of counterfeiting $1 and $2 bills was extremely low, so those two denominations retain their green color.