Blue
This sounds weird, but I think people with (naturally) red hair have some sort of affinity or connection with each other. It doesn't need to be acknowledged verbally or even if it did I wouldn't know quite how to say it, but there's a sensitivity or mutual understanding between people with red hair. Only 1% - 2% of the human population are redheads so we're all pretty closely connected if you ask me. I even think red is my favourite colour.