I find it interesting that xanh can mean blue/green. especially considering there is a type of colour blindness that merges these two. It makes me wonder if somebody important in the past had this condition and the language accomodated this.
The king says the leaves are blue, ok, green is now, blue like tree leaves. It wouldn't be the first time language has changed due to the desires of a powerful leader.
Different cultures divide up the color spectrum in different ways. It's not like the natural spectrum is clearly divided into six or seven sections, it's that Western culture has traditionally used it that way. Many Eastern cultures don't have separate words for green and blue, they consider the whole spectrum one color just like we consider sky blue and ocean blue to both be blue. There are other cultures out there that only have three color words: light, dark, and red.
This is actually very common in the world's languages. It's so common that we even have a word for it in linguistics... grue (green+blue=grue) In the late 60s, Berlin and Kay did a full study of basic color terms around that world and found a striking regularity. Here's a little bit more on the subject: https://en.wikipedia.org/wiki/Basic_Color_Terms:_Their_Universality_and_Evolution