1898 Congress of Universal Postal Union
The Universal Postal Union was organized in 1874 to standardize and simplify the international mailing system.  At its 1898 Congress, the UPU recommended green for the international single printed-matter rate (1¢ U.S.), red for the international postcard rate (2¢ U.S.) and blue for international single letter rate (5¢ U.S.). So the colors for these stamps were changed accordingly.