In my personal opinion the belt ranking system had very little legitimate meaning. I've seen white belt strikers KO brown belts, and seen the same with submissions. One aspect of the belt system I do agree with is the distinction between brown and black. This distinction is that, a brown belt is a technically versed practitioner of the art. Black means the user is versed enough in the eyes of his teacher to teach others.
So the answer would be no. A brown belt is not technically able to teach, his own teacher has not endowed on him enough respect to say, "you can teach what I've taught you." However, who's stopping him?
Sig courtesy of that photo-matic magician limba
I went out the way that I fight. I went out on my shield. That’s it. Done. --Chuck Liddell
My goal is to end my opponent --Carlos Condit