Why is American capitalized and asian isn't?
Both black and white are very commonly lower-case. For example in the Bloomberg article, they refer several times to "black person's" and "black." I consider it incorrect to upper-case a racial identifier if you're not going to upper-case them all. I alternatively could upper-case Black and White, however I also consider that wrong, and borderline stupid (again, my personal opinion).
If I were referring to a specific national heritage, I would upper-case. Vietnamese American. Nigerian American. German American. And so on.
I also would not upper-case "a Tall American" in description of someone. A person that is an asian American is not necessarily from Asia, asian is their race. A person is not from "black" or "white" either. It's as ridiculous to upper-case asian in this use, as it would be to upper-case Brown Eyed American or Green Eyed American or "the Freckled American."
But then you wouldn’t call a black Asian a black person, he’d just be South Asian.
Therefore since your use of colours is tied to the location of the person’s ancestors, Asia referring to the continent, oughtn’t it be capitalised?
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.
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.
Also, Google accused in lawsuit of excluding white and Asian men in hiring to boost diversity
https://www.usatoday.com/story/tech/news/2018/03/01/google-a...
We do say "White Americans", meaning Americans who are "white", which means people whose ancestors were Europeans (including Russia), or from Arabia, Persia, Afghanistan, North Africa, or some (but not all) other Muslim areas.
Then "Asian" includes everyone else whose ancestors were from Asia: China, Japan, India, Vietnam, Korea, Indonesia, etc.
When combined with nationality or geographical location, race is said first, so "White Asian" would mean someone living in Asia whose ancestors were European, most likely, though it's not a phrase I've heard.
proper nouns and derivations thereof are capitalized