How to add color to Cartoons

Ever since I got into Rammellzee (sp) and Graf I have considered my drawings to be cartoons. With the lines being my basic boxes and the area in between being my fill. So I am now interested in trying add more effects into the inside of the boxes to add more depth. I will probably be posting some graffiti tutorials soon.

Note to the Law: I have no intention of doing real graffiti. Too old and boring these days to risk incarceration. All tutorials are being used for Traditional Art purposes.