The criteria for the green dragons are too ambiguous

I don't really understand why the first four are okay but the latter five are not. It doesn't seem like just having a part of their body be green is enough, nor does it seem to be based on their type. I'm particularly curious about the difference between Flarefeel (the red one) and Ancient Naroot (the tree-like one); I wonder where the judgment is actually made. I thought the Folktale Brightwolf might pass as blue-green, but it was rejected. But if that's the case, then shouldn't Velvet Shimmerwing also be rejected? Since it's blue-green x crimson, right? I keep thinking that.