I think the American military presence is much less of a factor. The Army has gotten better about managing the young enlisted crowd, and anymore there's a greater likelihood that off-base shenanigans gets off-base punishments. And to be really blunt, GIs, especially enlisted, are viewed pretty low in comparison to other OECD foreigners by Koreans... just about the bottom of the developed-world heap. It used to be much worse, in terms of both how some GIs got away with stuff and general tensions between GIs and locals. In the 70s, there was a major riot in/near Itaewon, which started with black GIs not being provided the same services (as well as "services", ahem) as white GIs.

The colorism you brought up is interesting. Yeah, it exists, but I wouldn't say that it has a major impact all on its own. It's more or less something that kids tease each other about, and less directly, might play some role with the extreme importance of outward appearance in Korea (pictures on resumes, etc). It's also important to note that this colorism has just about nothing at all to do with white people; it's been considered a mark of beauty and class in Korea for centuries (kind of like it used to be in Europe, where you didn't want to identify as a tanned laborer).

The TEFL thing also requires a little more nuance. Those shitty job listings are written by the privileged majority, and they're also among the worst jobs. Korean Americans get the shaft there, but then again Korean Americans get easy access to a much better visa. Interestingly, Irish and other native speakers (white or otherwise) often have doors closed based on their accent- some schools specifically want a North American (most common) or RP-British teacher. We can connect all this back to ethno-nationalism (even ethno-linguistic-nationalism?), as language teaching visas in Korea are restricted to countries where some bureaucatic process has decided real native speakers come from- generally, wealthy(-ish) countries with a visible white population. So South Africans, regardless of what language they actually learned first, are in (including black and brown, though much less common in practice), while Indians and Phillipinos are out- again, you can see some blurry lines between race and economic status.