Certain motorcycle gangs wear specific colors to prove which gang they are a part of. They’ll often wear a specific color t-shirt when they’re together as a group, and they’ll have biker lifestyle patches on that say the group’s name. If a gang of another color comes into their territory, it will be a major issue. Colors and patches can only be worn by those fully sworn into the club, and not just those that want to be. Matt Sailor describes how this all works on the How Stuff Works website.