I was working in Chico, Ca. a while back and the GM at the store said there were counterfeit $100 bills circulating. The paper felt genuine, the ink was good, the thin embedded strip was good. The only way you could ID it was the green seal on the lower right corner, if you looked at it carefully you could see that the ink was flat and not raised.
He was there edumacating the employees.
The U.S. treasury, and just about everyone else, buys a special kind of black printing ink from a Swiss company that promises not to sell your tone to anyone else. The ink has special ultraviolet properties that are difficult to imitate. North Korea buys the same ink from them in Magenta. The North Korean “supers” use the altered magenta formulation.