I made a couple of doodle pages specifically for coloring, just to see if I could. The difference between these and regular doodles is that you need to leave plenty of space between the lines to permit coloring. I colored these digitally in GIMP, using just the Flood tool to make it quick, no special techniques. It was fast and easy and I had a great time.
Skip on over to Doodlebug if you'd like to keep up with my Doodle Art.