(Sarah of the Clue Crew reads from a printing press at the Bureau of Engraving and Printing in Washington, D.C.)  The color of the first ink used in printing $1 bills & the part of the sheet it goes on gives us this word that dates from 1862
However, the counterfeiter soon discovered that the colored inks then in 
use ... "patent green," for the color to strike through to the opposite side of a sheet.