Okay, since the whole thing is one color, this may not make any sense. The last step was for creating depth, now we are extending the concept with different shades of color.
Again, Nacchi's hair is pretty dark, so to bring it away from the background we will make it darker. Notice I'm shading her hair in layers... not all one solid color. It will look more 3D that way. Just shade in the direction that her hair goes to make it look like hair. You will get nice natural looking highlights that way.
Color also includes skin, clothing, and other things in the image. Try giving all parts a color, including white. I'm reserving the white for the triangle area we established in the very beginning, so everything else needs a color.