Originally, the green color imagery came from the idea of making the league a pay-to-play season. Real cash prizes to be awarded throughout the season. However, it seemed like we were going to have many new members this season and they probably wouldn't be too keen on paying for something new/unknown to them.
So we keep it free for another season. At least for now...
Nothing to do with the Hulk or the Avengers! Probably the ONLY thing not tied into that movie.