They are opposing gangs... large ones. One wears blue, and liked to use blue bandanas to identify their members. The other red.
There were cases in the 80s and 90s of people getting shot that were not in the gangs just because they happened to have a bandana of one of those colors.
Word got around the country to be careful about wearing blue and red bandanas.
They were (and I think still are) two very large gangs.