Unfortuantely that kind of thinking isn't along the lines of skin color. It's along the lines of any defined separation of ideas. All liberals think the same. All pickyourowngrouphere think the same. blah blah blah.
Because all humans make assumptions and create stereotypes. That's part of how we make sense of our world.
It's a natural extension of the things the human brain does to organize information. We start creating categories and making generalizations from the time we're toddlers. It's actually something we need to unlearn almost on a case-by-case basis as we encounter acceptions.