While your criticism is valid scientifically, this creature being only loosely associated with real biology only through name/title and pigment (or lack thereof in this case) allows for the fantastical and the magical to take place of reality. Especially since this is a dragon, and dragons do not have to follow science at all. So it not being scientifically accurate is no way a detraction or a fault of them as an artist. In fact, in my comment, I saw that color balance was more important than accuracy in order to bring out contrast and composition.
But thanks for the interesting factoids about reptiles and particularly chameleons.