Here's a quick peek into one of my sketchbooks: these little monsters were all drawn with a black Micron and colored with Letraset's AquaMarkers with the exception of the undead football fan. He was colored with Pitt pens; note the difference in luminosity. AquaMarkers take a bit of practice to use but in my opinion, the results are worth the initial learning curve.
For the curious, I work on my sketchbook pages bit by bit, sketching here and there, coloring things in over a period of days. Sometimes the pages have a unifying theme (like the one above) and other pages are just a compilation of random doodles that need to jump from my brain to paper. I compartmentalize my sketchbook needs just like my journals but unlike my journals, I get around to all 17 at least once a week. (I work A LOT in my sketchbooks...perhaps more than my journals.)