It's been a few years since I made this one... but I think I painted over a sketch with just some flat colors on separate layers; like, one a color, and then built up tones faded to blue on each layer. Merged it all down, added in some over-all highlights, tweaked, etc, tada. Something like that![link]
There's a screenshot for a pic I started at around the same time, shortly after I finished this one, but never finished.