1. To me, the use of the color black in the movie seemed to symbolize general frustration. Early on, Samuel Jackson's character jokes about wearing black to help absorb extra sunlight on a particularly sweltering day. On top of this, Pino, one of the more outwardly frustrated and angry characters throughout the movie, is seen dressed in black fairly often, and the riot scene is non-tinted and unlit, leaving the inside of the store and the people within nothing but silhouettes. Aside from scenes involving frustration and anger, black doesn't seem to appear at all.
2. I believe Mookie's action at the climax of the movie was more sympathetic than it is generally seen. When looking at the way the crowd reacted to Sal's perceived indifference towards Radio Raheem's death, it's not unreasonable to assume Sal and his sons would have ended up viciously killed in the ensuing riot, if Mookie had not drawn their attentions towards the pizzeria instead. Beyond that, it's understandable that Mookie would react the way he did even disregarding that... witnessing a murder could definitely shake him enough to want to incite a riot in retaliation.
3. I think that the movie was not a political movie. While dealing with various arguments popular at the time, the movie takes no stand behind any particular one, and focuses more on the out-of-control confrontations that have little to nothing to do with said arguments. By the time the various fights climax, the people involved have forgotten whatever argument got them involved in th first place, and involve themselves more on a personal level-putting on a show for peers, redeeming one's hurt pride, and the like.