From my prior knowledge of melodrama, I always assumed it meant something exaggerated or overly dramatic. What I've grasped from out in class discussion and from this paper (although I'd say I am slightly confused) I'd say that my assumption holds true. That being said, one Harry Potter passage that I picked out was from Harry Potter and the Sorcerer's Stone. The passage reads as follows:
"He brought the umbrella swishing down through the air to point at Dudley - there was a flash of violet light, a sound like a firecracker, a sharp squeal, and the next second, Dudley was dancing on the spot with his hands clasped over his fat bottom, howling in pain. When he turned his back on them, Harry saw a curly pig's tail poking through the hole in his trousers.
Uncle Vernon roared. Pulling Aunt Petunia and Dudley into the other room, he cast one last terrified look at Hagrid and slammed the door behind them."
In this case, I suppose the Dursleys would be the bad and Harry the good, however, it doesn't indicate that with colors or anything of that sort. I would classify this as melodramatic, though, because it seems like such a dramatic scene. Things happen so quickly and they are outrageous events. Each verb like "roared" and "slammed" are rather dramatic words that JK Rowling chose to use in this scene. If I were to rewrite this in a non-melodramatic manner, I would write:
"He brought the umbrella and pointed it at Dudley - there was a flash of light, a loud noise, a squeal, and then, Dudley was dancing, his hands over his bottom, yelling in pain. When he turned his back on them, Harry saw a curly pig's tail poking through his trousers.
Uncle Vernon yelled. Directing Aunt Petunia and Dudley into the other room, he looked at Hagrid and shut the door behind them."