An idea popped into my mind about oni and dragons. We all probably know or at least heard the metaphors: "heart of gold" or "heart black as night" (in my country it's "black as tar") . Those also exist in Ninjago, but no one knows their true origin. The origins that are tied to oni and dragons. How? Because those are the colors of hearts of dragons and oni. Dragons are associated with Creation which is represented with gold/white colors, and oni are associated with Destruction which is represented with black/purple/neon pink colors. Plus the oni, based on resurrected Garmadon's design, seem to have purple blood, so it wouldn't be too far fetched to asume that dragons blood is also close in color to the element they are associated with. Because the shade of their blood is different their organs also have different color. Hence dragons have yellow heart and oni have navy blue/dark purple hearts. Them being the symbols of good and evil caused people to start to associate those creatures with traits they saw in each other. This lead to the birth of various metaphors. After all, what would be a bigger praise than telling someone they're heart is just like dragon's. The embodiment of pure goodness and light. Or what would be a bigger insult than comparing someone's heart to the devil's. The walking embodiment of evil and darkness.
