Mongol color is the worst for me.
I also think Poland should have Austria's colors, but I digress there.
Japan's colors are the exact opposite of Austria's colors, so it's not really necessary to go down that road. Essentially, their white is more pronounced than their red. Poland may be equal parts red and white, but having slightly more red but still white is better than red and even darker red.
Aside from that, England, The Netherlands, Sweden, Arabia, and Indonesia, are close, but still need slight tweaking to really fit. I've thought way to much about this.
No, I mean giving Poland the exact colour palette Japan has now - i.e. with white as their main colour rather than their current red.
I'm just not sure why you singled out Austria rather than Japan for the change, when either would work equally well.
Also, The Wonder's of the Ancient World scenario had some really good colors s well, that are not used in the main game! Not really sure why they never used Sumeria's anywhere else, along side The Hittites'.
Poland is my least favorite, the red is too sharp and seems to clash with absolutely everything, especially given the amber-ish tone the interface and map has. It just seems too dark, aggressive, and uninviting.
Considering you meet with Casimir in a dungeon, I don't think they were going for an "inviting" theme with Poland.
My favorites are Portugal, Shoshone, and Indonesia. I dislike Siam's ketchup & mustard theme and Inca's colors are just a bit too loud.