I'd imagine many, if not all clans will have some sort of mark/sign/appearance that signifies that they are part of that clan.
Also, if a person starts out without a clan than joins a clan where everybody has Red eyes, how will he join? Or are we able to change our appearance when joining a clan?
I'm not entirely sure how much we would be able to change appearance wise. This is the only thing I can think of on the top of my head: (this is pure speculation)
When joining a clan that has an appearance trait or any other sort of trait associated with it, you will automatically be given said appearance trait upon joining. So in your example, you would get red eyes if you joined the clan.
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