So I started out with a sketch, which turned into a lineart:
User Image - Blocked by "Display Image" Settings. Click to show.

Then I colored it with a more cell-shaded style:
User Image - Blocked by "Display Image" Settings. Click to show.

Then highlighted it:
User Image - Blocked by "Display Image" Settings. Click to show.

All mouse-drawn in Photoshop CS4 Portable. Ended up taking slightly over three hours.