Small dithering tip: Some of your colours are only used in 50% dither ( there is no solid patches of those colors ). When dithering most of the time the intention is to create the illusion of a colour in between the two used for dithering without the need of adding a new one to your pallete. So if your highlights are only used dithered then you may consider to just choose a colour between the dithered ones and create a solid patch.
Also remember the effect that dithering has on the material of the object, the stronger the dithering the less smooth the material will look.
I would say you shouldnt bother dithering until you have all the volumes defined with solid patches of colours, then use dithering just to blur the edges of those patches.
About the eye: Remember that the eye takes shadows and is lit in the same way as any other thing, exept it also has gloss. Its white, but not 100% white. Shade the eye like you would shade a sphere, then cast shadows onto it ( from the eyebrow thing maybe ). Then for the glossy effect add some reflections of bright things in the surroundings.
here's a fast edit:
