Last year i have worked on Go, rGo, rGo+Metal oxides and so on. I have prepared graphene oxide by improve method (Hummer method) and i got black ppt in all experiments. I am sending you JPEG images of my GO in attachment. I dont know why you got brown color?
The precipitate should be dark brown going into darkish but if you redisperse the powder in water the dispersion should be brown. If your dispersion is black in water and is not stable (it will precipitate again in few time), it means that the powder is not oxidized.
In our lab, we obtain brown GO. I also encountered some cases when black GO was obtained. I tried to analyze the difference between brown and black GO and i found out that black GO has fewer functionalities (based on FTIR) and weaker XRD signal. It is possible that the black GO was produced when mixing is insufficient.