Nodules are red, violates are blue
I was today years old when I learned why the nitrogen fixing nodules on the roots of legumes tend to be red when you open them. It has to do with the Great Oxidation Event, which happened around 2.5 billion years ago, and is directly connected to the reason why our blood is red.
(This article also exists in Spanish: link)
Nitrogen is naturally scarce. It might make up the bulk of what we call “air”, but this nitrogen exists as N2, and most organisms have no clue what to do with that. “Most organisms”, because there are a few bacteria that have evolved the capacity to capture nitrogen from the atmosphere, and turn it into molecules that are biologically useful, like NH3.