A quickie. Though, I had to think a little bit about its creation. In my art jam I said that a good idea could win over great art. And this is my example of a good idea.
It is my understanding that the Indigo Lanterns are healers with no specific allegiance. They are powered by compassion. (Which is why I tried to give her an "Are you OK?" expression.)
Natu is a doctor, first and foremost. That kind of profession seems more associated more with compassion than with willpower.
Moreover, her Green Lantern outfit echoes the tribal vestments of the Indigo lanterns, and I heard somewhere that the Green Lantern uniform is influenced by the Green Lantern.
Finally, she and Kyle are breaking the law, and will eventually be caught and punished. Which may include but not be limited to getting kicked out of the corps. Considering Kyle's track record, this is the best thing that could happen to her.