What ink is used on US dollar bills?

Answer 1

To discourage counterfeiting, the kind of ink that is used on a dollar bill is not information that's available to the general public.

The Treasury has used green ink since the nineteenth century. It was originally developed because the color was difficult to reproduce using the primitive copying techniques (mostly photolithography) available at the time. The color eventually became almost traditional (see the term "greenback" for the US dollar) despite advances in copying technology that made it easy for counterfeiters to reproduce.

By the late 1970s it became clear that more than just green ink was needed to prevent widespread copying of US bills. Even so, the Treasury, Congress and the general public resisted any redesign efforts. Some opposition was based on the above-mentioned tradition and concerns that changing designs might be interpreted as a signal of monetary instability, while others were far more wild-eyed; e.g. new designs would confuse senior citizens, redesign was a "European" idea, and of course the ever-popular insinuations of a communist plot.

It took until the first years of the new century for a full redesign to be authorized. The current series of blue and peach colored $20 bills was released in late 2003 (with a 2004 date, interestingly), followed by $50, $10, $5, and $100 in that order.