What should I wear to El Clasico?

I am attending a Barcelona vs Real Madrid football game and I would like to know what is traditional attire....I am an American, going to the game with my English boyfriend. We're sitting midway up behind the goal. I know very little about soccer but will be supporting Barcelona (Don't worry I plan on reading up on both teams before the game). I just want to know what attire is appropriate to wear to the game! Thank you very much!