I did a edit to show you some things i would change.

The first thing i did was throw the pic into color so i could get the hair to
look right and i looked up some ref photos.
Notice how i removed the dark shades from the hair and only used
them a little bit.
In the ref pics i was looking at most of Nia's hair is defined using color
difference and not Depth.
So drawing it in color helps a lot.
Then when i got the hair to match the ref pic I removed the color
and replaced it with the gameboy style colors.
As you can tell i re did the gameboy colors you were using to
have more and even contrast.
I hope that helps you out.