From the 21st Mecha Sketchbook, this is a shot at cel-coloring in the GIMP. A lot of highs and lows as time went on; I could sympathize with the machine's attitude right now...
Most fun: the LOL SHOOP-DA-WHOOP LAZERS!!11 and the elbow power plants' exhaust flames. Indirectly referenced the SR-71 Blackbird's engines on afterburner for color ideas (here).
Least fun: touching up the original lineart. Blocking out highlights and small shadows with the Bezier tool. Perspective transforms on the main shadow. Watching the laptop constantly crash from running out of RAM. Complaining about things like these... -_-'
It's great for making very smooth curves as well as defining exactly where edges should be, but it all takes a long time since I have to do it for every shadow.
But, you could always use the brush tool.
Laser beams + Mecha = win.