Going back to the original topic, there is a strong tendency among humans to categorize ourselves as "us" and "them", and, given that unlike other organisms we obtain most of our information visually, we tend to do it via social cues. This tendency is difficult to overcome, unfortunately, even when these visual cues no longer mean much. One way to overcome it is by being surrounded, particulalry when growing up, by a wide diversity of people, and ethnicity (or colour) will cease to be the factor that separates us (people will likely find other methods to separate "us" from "them" but hopefully these catagorizations will be more ephemeral and not pre-determined at birth).
"Them" and "us" is as categorically real as "you" and "me". There really is a "them", and there really is an "us". In fact, there's LOTS of "thems" and lots of "uses".
The problem isn't making distinctions or having categories. The problem is when people use those categories to make judgements beyond the qualifying criteria.