This was a book I read when I was about 13, around the year 2002. I borrowed it from the school library at the time and really enjoyed the story (at least, I think I remember enjoying it). I've marked it as YA, but I don't know if it actually was. Anyway, I've completely forgotten what it's called, and it's honestly bugged me for probably the best part of 8-10 years.
As I remember it, the story centred around a war/conflict point between two fantasy nations. Both races were humanoid, with the first being potentially elf-like (knights, armour, swords, beautiful hair(?!)) whereas the second were more... savage? Not so much armour, less swords, and wielding weapons I distinctly recall being described as something like crude black powder weaponry. I wouldn't go so far as to call them guns as we have known them throughout our own history, more like... clubs that exploded projectiles.
That's about all the plot points I can really remember. But I do seem to recall that the savage race were invading the other, and that they potentially were from across the sea.
As for any further detail? Green, Jade, Moon are keywords I seems to recall from the title.
It is not The Green and The Gray by Timothy Zahn.