There are two layer modes in Paint Tool Sai that really come in handy with cartoon coloring: Shade mode for shadows and Luminosity mode for highlights. I've seen them in action on a few YouTube videos and was impressed with how easy it was to add shadows and highlights on a project.
The kicker is that I use a Mac Pro and would have to run SAI within a virtual machine if I want to use it. However, I shouldn't have to since I have Manga Studio 5. The problem is that there isn't a great deal of information about Manga Studio 5 in relation to Photoshop or SAI or any other paint/drawing program. The information I did locate wasn't very helpful ei