I reasoned that if plants are made of minerals and water, then the geometry of those minerals would determine which bandwidth of light would be reflected, and which would be absorbed.

I can only think of a carrot as another plant that would be considered a deep feeder like dock, though there are surely more that I haven't learned of yet. There are lots of varieties of dock here, though I don't know them all by name.
The greens in the soup should count, some vitamins might be lost in cooking, but the minerals that they contain should still be there if they are cooked there in the soup, I think.