Ah..I think this cover is more challenging to colour than people realise. I actually think this is a great drawing by Kishi, and, believe it or not, I think it is one of his most expressive character portraits yet.
Call me crazy!
Having said that, I think it would be hard to make this image come to life, so well done for what you've done so far! I wonder if he wouldn't look better with bolder, stronger colours? Maybe some brighter tones? What do you think?