Colonel Green was a career soldier and the head of security and operations at the Los Alamos Research Laboratory before the war. He was a hard man who expected things to be done properly the first time and would not tolerate people complaining. Not much has changed in the two-hundred years that followed his ghoulification.
Colonel Green was not one of the original twelve volunteers to traverse the wasteland in search of survivors. He thought it was a futile cause and wanted to stay back and build up a community at the Reservation, of which he would be the leader. His dream to rule over a tiny empire was shattered when Willem Clark returned from his year long journey with nearly three dozen ghouls. It was like the Messiah returning from a holy pilgrimage. Colonel Green knew he could not compete with Willem after that. Instead, he offered his military leadership skills to Willem and formed an elite group of ghoul guards. Under his strict command, he and his ghoul guards became every bit as efficient of warriors as any Enclave or BoS soldier. With the added bonus of high powered rifles, caravans and raiders who know about the Reservation, knew not to mess with Colonel Green and his crew.
The one snag about Colonel Green is that he is extremely envious, not to mention jealous, of Dr. Willem Clark's commanding power over his fellow ghouls. In addition, Colonel Green does not agree that taking the fight outward to potential enemies is a very good idea (the whole dirty bomb scenario). Green would prefer to build up the ghouls' arsenal and take a defensive posture – bring the fight to us, if you dare - type of thinking. If he could figure out a way to undermine Willem and seize control, he would be a very happy ghoul.
Colonel Green has not appeared nor is he mentioned in any published game. He was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if he will appear in any future Fallout game.