@Vincenzo -- There is no problem with an officer breaking up that fight in your scenario. That, however, is not exactly what this article addresses. The term "color of law" comes into play usually when someone does something wrong.
In other words, an off duty cop breaking up a fight is fine. A cop saying he won't charge a criminal in exchange for cash, however, is not. A cop using his authority to break in line at a restaurant wouldn't be OK, either.