It is amazing how well you can shade and blend colors together, if you work quickly enough, with Sharpies.
Using one color as a solid base, and then later going over that base cooler with the same Sharpie makes creating soft shadows easier, and they blend into the drawing much nicer than using an entirely separate color. I used this technique in a couple places here.
Twas indeed a fun drawing, and it is always much more difficult to create something almost entirely from your imagination (as apose to basing your work off of a photo) but I really enjoy the loose, simple properties I get trying to do so :)