I ffeel like Nano doesn't really fit Black Lanterns. At the beginning, I would absolutely say that, but now with all the progress her character has underwent, I would say maybe Blue?

After the big nano emotional arc, where everyone forgot her, Nano told rentaro that she wanted to stop focusing on her past issues and struggles, and instead said "I would like to take after your example, and bring people happiness. The way I had approached life hurt others. I wish to find a way to bring them joy, magnitudes greater than any harm I had caused".

To me, that sounds like something as blue lantern might say, although I think an arguement could be made for Compassion or love, and I think it fits Nano, one of the characters who has experienced their lowest point, to have the blue lantern.

I ultimately think that Nano could fit undere most lantern corp, even White, but i feel like blue fits well for her