The significance of Sal, his store, and the bicyclist having the color green is not entirely clear to me. My first thought was green being the color of money and wealth, which at first seems to make sense since Sal owns the most successful business of all the main characters. However there is no indication of the status of the bicyclist, after all he does live in the same neighborhood.
2. I was surprised by Mookie's actions at the end and I did not feel like he was justified by any measure. Throughout the film he seemed to play under the guise of being one of the more responsible characters because he was one of the few with a real job, but by the end it becomes plainly obvious that he doesn't really care about his job so much as he does getting paid regardless of the work he does (or doesn't do). Even after he incites the destruction of Sal's Pizzeria he expects to get paid for his previous day of work, which was half-assed at best.
3. I would agree that Do The Right Thing is about arguments in general, but to me that doesn't eliminate the presence of politics in the film. I think the Spike Lee means to show how relatively minor personal grievances can easily and rapidly escalate into violent conflicts when not reconciled appropriately, and he does so in the context of race.