I added white=green, then green=white to my .tn5250rc file, but I still
have a black background with white text
Ummm... The typical color of text sent by the system would be green.
So if you map green to white, you SHOULD get white text. In your
config, you don't appear to have re-mapped black to anything else.
So I guess I'm not following you?
The other possibility might be that you're not using a color xterm...
honestly, it's been years since I saw a non-color xterm, but then, I
rarely work in anything else besides FreeBSD so maybe this is still
common on other platforms, I don't know...?