I can't access it right now, but what I did was this
White edges
White
Black edges
Black
Color
Lineart
Line art is a normal layer, color is multiply. The other layers are partially transparent, and are done with a big soft brush. I use selection to isolate each individual part to shade, then do each shading layer on that part. I use 2 layers for each type of shading so I can somewhat simulate pen pressure. I have to do it this way because all I have is a mouse, and can't do things with too much precision.
The problem is that sometimes the white and black layers touch, and it creates a gray that discolors the image. I want to see if there was a way I could do this so that when white and black meet, they just cancel each other out and just act as transparent, and do so porportionately