See This Link : http://en.wikipedia.org/wiki/Agent_Orange Agent orange were the nicknames given to the herbicide and defoliant used by the United States Armed Forces in its Herbicidal Warfare program...
The military often color codes their ordnance. An aerial bomb or artillery shell marked with YELLOW letters means explosive. Agent Orange was shipped with an ORANGE marking on it.
It was a chemical based de-folliant used to kill jungle growth, so the North Vietnamese had less cover to work in and set up ambushes. It was also used in the South to kill growth so the Vietcong...