I used to do manga style cel shading, and the way I did it looks similar to what those tutorials do. I'd do a layer with all the flat colours, then make a new layer for the shading. Then I'd use the polygon lasso tool (but you can use any selection tool, just whatever you prefer), then fill the selection with my shadow or highlight colour. I've seen some pretty cool cel shading done with gradients too, but never been able to reproduce them myself. Unless it wasn't gradients after all.
Can I ask a similar question here? I've been wanting to recreate the the style of colouring in the pic below for a while now, but I've never managed to get it right on the occasions I've tried. Does anyone know how it's achieved, or at least how they would do it?

I miss Witchblade actually, must try and get myself some more.
http://www.artfulscruff.co.uk |
deviantartPlaystation Network user ID: artfulscruff