All of you live in suburbs...

So clueless..
Anyways..if you come to a city like Chicago/L.A/New York, and live/go to school in these (and some other) cities, you will find a thing called Gangs. And some of them have "Gang Colors", and if you are wearing their "gang colors" and walk on the street, and get confronted by these "gangsters" you will get beaten up. Simple.

For those who do not know of these terms (Suburbians)

Gangs - a group of persons working together (2) : a group of persons working to unlawful or antisocial ends; especially : a band of antisocial adolescents

Gangsters - 'ga[ng]-st&r - a member of a gang of criminals

City - Pronunciation: 'si-tE - an inhabited place of greater size, population, or importance than a town or village.