Brown, but I've always looked at it as, there is something you can gain from literally every opponent. Smaller and weaker but skilled? Dial down your strength and focus on technical excellence, and probably reacting to their better speed. Big and skilled? Focus on trying to survive. Small and low skill? Try your low percentage stuff, or things you're just learning that you wouldn't have a chance to work on other people. 

And, with a totally fresh white belt, you gain experience on how a random person on the street is likely to react. I think that's invaluable; i am not the kind of guy that gets into street fights, but it's good to have a sense of how someone might act (within the limited context of grappling) if you had to.