I think I have a few notes that may be of use to you. Some things I would have approached differently than you, some the same. First understand that I got nothing but respect for a challenge of this type. M
-The brown is way too strong. It can be that saturated brown near the horizon, but you need to fade it out as you paint away from the horizon line (HL). It should fade to a more neutral, faded appearance.
-If you want a true mirror-like surface then your ground needs to fade towards white as one moves away from the HL. The sky fades to white as you approach the HL. If u don't want a chrome mirror finish, and u prefer a reflective steel for instance, then fade to gray instead of white.
-Many of your HL placements aren't correct. Take the HL that travels neatly along the inside of his legs. This is incorrect. The legs are treated like cylinders. As such, the HL always travels along the X axis, which you appear to be following. However, where you go wrong is that it looks like the legs are broken down into several pieces, rather than one uninterrupted HL. Rather than one vast horizon, I think you should have a few per leg. I think you followed this, generally, on the upper parts.
-Make sure that you are edge highlighting the different pieces too. Each panel should have white edge highlighting.
-Bend some of the horizon lines at wierd angles. Add some areas of interest, like hills or mountains on the HL. The face, for example:the horizon should bend drastically upwards towards the ears. Too flat as is.
Hope this helps. The blue looks great by the way, don't change that at all.