It’s just my personal taste, but I think green (to resemble centenary USD notes) suits US-NBT better.
It’s just symbolism:
US-NBT: green (USD notes)
EU-NBT: blue (EU’s flag)
CN-NBT: red (China’s flag)
X-NBT: dunno… maybe black (for neutrality???)…
NSR: dunno… maybe orange
In the future (I hope):
AU-NBT: yellow (for gold)
AG-NBT: grey (for silver)
P.s. Nubit’s yellow reminds me of Gold (AU-NBT?) so much… 