Unfortunately I didn't make a tutorial when painting this piece, sorry.
But I can explain my technique: What I do is I make the greyscale shading first. Just put a new layer under the lineart and build the form in shades of grey, and concentrate on the source of light, form and dimensions. Then once the greyscale is done, make a new layer to the very top, set it to overlay-mode and lay down colors as flat areas. Then make a new layer on top of all others and start painting details, fixing, detailing etc.