Joe Colton was the way that our good old friends from the Adventure Team fit into the mythos of the G.I. Joe Comic Book series.
His bio at the G.I. Joe Comics Home Page gives the following information:
In 1969, Joe grew tired of a soldier's life and took a well-deserved sabbatical. He began to travel the world and explored some of the most exotic and dangerous parts of the world, having a number of adventures. In 1970, he became a part of the civilian group known as the Adventure Team. Throughout the next decade, Joe and his team worked for private groups, serving as guides and rescuers to archaeologists and other scientists working in the field. The team even worked for various government agencies on occasion. The Adventure Team disbanded as the 70s came to a close.
Now that is the Joe I remember!