Years ago I read about the filming of one of the Planet of the Apes movies. The studio put out a call for extras to play various types of apes...gorillas, chimpanzees, orangutans, etc. The extras showed up and put on their costumes, which they could not take off until the end of the day.
At lunch in the commissary, they noticed something remarkable. All of the gorillas were sitting with other gorillas. Chimpanzees were sitting with other chimpanzees, etc. There were even cases where two friends came in together to be extras and were assigned different types of costumes, and instead of sitting with each other, they sat with strangers who had the same kind of ape costume!
This was a case where the extras had nothing to go by except costumes. They had no idea who was white or black, rich or poor, pretty or ugly, young or old. They only saw costumes...and they segregated themselves by costume, without even realizing they were doing it.
It’s human nature! Time people got over it.
As to your comment, and Liz's above about whether these behaviors are hard-wired, I had an interesting experience. It has something to do with the way people in the UK tease and bully kids with red hair and/or freckles, coloring that they call "ginger." I always thought it might have originated in England as a prejudice against the Irish.
For several years I dressed teddy bears in patriotic neckties and took them to an army hospital here in the US, where there was a donation table on weekends in an area where soldiers and their family members passed by. I purchased bears that were available in a range of "skin" colors, including white, beige, tan, golden, ginger (like red hair), brown and black; and usually took one of each every week.
There were white, black, mixed race and hispanic (brown) soldiers. Sometimes soldiers picked a bear that matched their skin color, sometimes not -- or maybe they took the one they liked best of whichever ones remained after other people had chosen a bear. But one thing happened almost every week: the "ginger" bear was left over, or last chosen. Eventually I stopped even supplying them. And I love red hair!