This is how I shade!

Step one: Draw anything that is in one piece (not a entire art piece worth of lineart)

Step two: copy that onto a new layer.

Step three: Choose a color and fill in the bottom layer

Step four: Choose another color for the shading

Step five: on the very edge, make a scribble shading on the bottom layer

Step six: Turn the opacity down really low and do a scribble shade up to the point where no shading will be

Ok you know the rest of the second color thing, Then you turn down the opacity to about half on the first color and overlay as many times as needed to make the shading look good

--finished--