> I'm trying to think of what a fair comparison would be.

You should be able to think of a dozen examples off the top of your head. Virtually every society has minorities and immigrant groups (which have nothing to do with colonial history).

> I do think there's a meaningful difference between a dominant/imperial power that (begrudgingly, slowly) allows room for its own citizens of diverse racial backgrounds

Why would a dominant power ever make room for people outside their in-group? Where does that notion even come from? That's not how most societies work. Some multi-ethnic empires in history showed various degrees of tolerance for outgroups (e.g. Muslims that ruled over the Indian subcontinent imposed jizya on non-Muslims only some of the time). But you have to go back to the Romans to find a major power that allowed outside ethnicities to rise to the uppermost reaches of society (without being conquered by outside groups).

You can't explain the unusual inclusiveness of American society by pointing to anything minorities did. Minorities always advocate in their own interest--that's commonplace, but almost never works. The Uyghurs can tell the Chinese "we don't want to be oppressed" all they want, but that's not persuasive to the Chinese because that's just an expression of self-interest. It's not contrary to the self interest of the Chinese for the Uyghurs to be oppressed.

The unusual thing is the dominant group actually giving up power voluntarily. For that to happen, there must be something in the dominant culture to which minorities can appeal, something that can be used to persuade the dominant group to give up its own self interest.