I colored this robot in a few different ways:
I used a gradient to do the bulk of the coloring on this robot, then I used a screen brush to add some highlights.
I colored this robot by hand. Using my red brush pen, I feathered lines over the black feathered lines.
I colored the high-lights, the shadows, and the mid-tones in photoshop, but I used only the lasso and bucket tool, so you can really see where each color starts and stops.