Awesome Jonathan! I particularly like the Armored Core esque of the main torso and that crazy cool head full of vents! The red arms against a relatively plain "army green" scheme is a cool touch that adds a neat bit of character as well. Really doing a good job on these, I look forward to more as usual!
I uploaded a video with the process here: youtu.be/4ct7E7lMmIw I start with a midtone and then "paint" with light thinking about where it comes from (just like an arrow). I really recommend you "Color & Light" from James Gurney, it really helps you understanding how light works.
I have that book and read it... it is great but as you can tell from my art, I have much to learn about how exactly to plot my light sources, and how the position and strength of light affects the rest of image... maybe I will re-read it Thanks for comment, love these designs! Great video too