very good question. In my experience violent fans only target the "enemy" fans. they don't care about colour/race or anything, they're just primitive thugs who want to beat up someone (anyone, really). most of them don't even care for the sport, they're just there for the violence.
for example. I was at a game of the HC Lugano (swiss hockey team) this winter. there were guys there (from italy, I think). that had now idea at all about hockey. they just came, shouted some stupid chants, and tried to stir up trouble everywhere :rolleyes.