BP is providing this live video from one of the ROVs assisting in the “top kill” procedure:
It is my understanding that the brown fluid is drilling mud venting from the damaged riser pipe. The mud is being injected downstream into the blowout preventer, and the hope is that while much of it is leaking from the riser, the majority of the pressure is downward into the well.
Update (05/27/10 9:02am): Tentative reports suggest the top kill effort was successful in stopping the flow. If this is confirmed, the next step would be cement the wellhead.