> I'm reminded of the experiments where camp kids were randomly given blue and red shirts, and it took them less than an hour to form in-group/out-group dynamics and tribal loyalty based on shirts alone.

You slightly misremember that (Robbers cave study). The effect is even more powerful: all it took was the seperation into two groups and being made aware of the existence of the other group for them to develop an identity and name for their respective group.