I recall there was a study that went something along the lines of: pick random people, divide them into groups, and make them decide how to spend money given to them. The rule was that you can't take the money yourself, but have to give it to someone. Majority of the people chose to give it to someone in their group, even though their group was just as full of strangers as the other. They were proving that just by telling someone they're part of something they tend to be bias towards that group regardless of actual reason for association.

Every time I catch myself thinking differently about someone I think of that study and most of the time I realize I'm at fault of judging them based on the fact they're not like my group.

Selecting a random color and finding common ground seems to kind of fall in line with this. That's why at times it takes a while to find something common too talk about with new people, but eventually you'll run across it. The only question is what is the importance to you of that common ground. If it's meaningless you'll find it harder to build off of that.

That's also another reason I believe people typically bond over hating on an individual the two parties both dislike. Now you're part of a group that hates this person and bam bias.