[stagbeetle]

American is a proper noun in this case. White and asian (or should I rephrase it as "Asian and white?"), are used as descriptive adjectives here, also. If it were rephrased as "Whites and Asians, from America," then it would be capitalized because all three are now proper nouns.

[dragonwriter]

White and Asian are proper adjectives in the sentence, and would normally be capitalized.

https://en.m.wikipedia.org/wiki/Proper_adjective

[stagbeetle]

Proper adjectives are for British English only. Even so, I don't see them often nor do I know of any current English grammar texts or thought leaders that recognize proper adjectives.

[dragonwriter]

> Proper adjectives are for British English only.

No, they aren't.

> Even so, I don't see them often nor do I know of any current English grammar texts or thought leaders that recognize proper adjectives.

Chicago Manual of Style and AP Stylebook, two of the leading American style guides, both recognize proper adjectives.

[stagbeetle]

For the British English claim, I have no source nor can I find one. It's what I was taught long ago and that the Fowler brothers use it implicitly in all of their series.

However, I do have a rebuttal to your style guides. They are only that: descriptivist style guides. They clash with one another and are nothing more than another dialect of English. Whereas Loweth's English Grammar, the recognized beginners in English prescriptivism standard, does not recognize proper adjectives.