I would have the following suggestions:
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As for images of JPNBT and KRNBT, the obvious choice would be to use as you suggested the colors of the corresponding flags, as it was, it seems the guideline for the previous ones.
So for JPNBT, a mix of white and red, for KRNBT, a mix of red and blue would be the natural choice but it could be a bit difficult to recognize.
Using only one single color, i.e red for JPNBT and say blue for KRNBT would be better but it is already taken by NSR and EUNBT.
So I would suggest to rely on the colors used by the corresponding national main airline companies instead. Because usually national airliners are pretty symbolic of the corresponding countries.
For JPNBT, it could be sky blue used by All Nippon Airways or the grey used by Japan Airlines.
For KRNBT, I would suggest their sky blue.
Or we could use alternatively the colors used by the national soccer team jerseys.