It's badly described, but it's a reference to structural color. Engine oil isn't rainbow-colored, but it can produce rainbow reflections due to thin-film interference. Likewise, some animals have features that appear blue, but wouldn't yield a blue pigment because the color is the result of a particular microstructure.
In some languages (such as Vietnamese XANH) blue and green use the same word. I always wondered if it was because blue was so rare (other than the sky), why make a new word a color you never see?
> verdant: (of countryside) green with grass or other rich vegetation.
> "verdant valleys"
> "a deep, verdant green"
Learned a new word! I wonder, however, if this word is ever used outside of SATs.
This is meaningless nonsense. If the light reflected or refracted by them is blue, then they are, indeed, blue.
https://en.wikipedia.org/wiki/Structural_coloration
https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...
Verde is not one of them though.