Goody, a new question! You're probably right, it's been discussed, but who cares.
I know what my first reaction to John's remark was ... he didn't want to be looked down on as just a tag-a-long friend, a nobody; he wanted to assert that he was an "equal partner" in Sherlock's business.
However, once I discovered the "Johnlock" theories, I thought maybe it meant John was already tired of people assuming he and Sherlock were a "couple", and said "colleague" to make it clear that they were not romantically linked.
I also seem to remember other people saying it was because John still wasn't sure if he wanted to be a "friend" of that oddball Sherlock, and that's why he corrected it to "colleague." A lot of people interpret Sherlock's expression after John says that as being hurt by John's rejection, so that theory would make the most sense in that case.
It also occurs to me; that's how Sherlock introduced him to Donovan, as a "colleague". So maybe John was just following his lead.
Hey, we're Sherlockians ... no easy answers around here!
Oh, and John probably wouldn't have understood by then, that he is undermining Sherlock by correcting him. I don't think they were supposed to know each other that well yet.