I think compared to the reference, your colors are a bit all over the place, especially cheek/nose and jaw areas, on both sides of the face.
Also take care notice from the original the dithering and how colors work next to eachother. Your dithering is quite messy in the same areas where colors are messed. I would first shade the face with clear areas of color (like Indigo's portrait) and then break them with dither to get the antialias/texture/etc. you want to achieve.
Also check the antialias on his left side of the face (because yours in against a black background and not transparent, due to the hair) and his ear and hair. Even when Helm didn't use any specific AA on the hair, its shape was by using angles which required none, but yours would benefit from it. With the hair you don't need any new colors for AA, just dither the edges which need the AA.
Nevertheless, nice progress, but not there just yet
