In S2 (I forget which ep) Colby tells David that his college degree kept him off the frontlines- I'm inclined to think that made him an officer. But in The Mole, Dwayne Carter refers to him as PFC (Private First Class) Granger.
Personally, I'm inclined to go with the officer theory- Colby needs that college degree to enter the FBI, not to mention having him go to college after getting out of the Army contradicts previous canon plus messing with the timeline, since he would have been out by the time American forces went in. Not to mention I don't think he would have been as affected in S4 had he been out of Afghanistan six years instead of going directly from the Army to Quantico, but I could be wrong.
What do y'all think? Do you personally go with Dwayne's version and have Colby as an enlisted man, or do you prefer to have him be an officer?