[quote]How ironic that the amount of ancestry necessary for benefits in the USA ranges from 1/4 to 1/16, depending on the tribe.[/quote]
that's mostly because metis despite being mixed people are considered separate/unique from either whites or amerindians.

[quote]The British Isles and many other examples didn't exist completely independently of other biological groups, like a Mars colony would likely be. [/quote]
Enough for your hypothetical inbreeding issues to have surface over that time, particularly with the lack of the technologies and record keeping needed to avoid it. Anyway, going by 

[quote]After all, one of the assumptions of this thought experiment is that the colonists are objectively grouped by ethnicity.[/quote]

that may well be true but it wasn't said they'd be selected solely by ethnicity.