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 interf

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. https://en.wikipedia.org/wiki/Structural_coloration https://en.wikipedia.org/wiki/Structural_coloration