I'm still pretty sure that Michaelas nickname „Grüne“ means „the green one“ in english. „Die Grüne“ is a noun and it's just the term for a green girl. If it was a boy it would be „der Grüne.“ the color green just means „grün“ in german and it's an adjective.
I'm pretty sure this is correct because german is my native language.
I changed it back to "green one" as you might have noticed, I realized that I was wrong. Thank you for the clarification, however! My only experience with the term "grüne" has just been that it's an alteration of "grün", I didn't realize it was a noun in and of itself.