Shading
From this point on. The screen shots will be from Photoshop CS3. Yes, it took me -that- long to get around to writing this section. I have procrastination down to an art. I should eventually maybe pose for a statue sometime. Anyway, the process is still the same. k?
By now you know the drill. Save! and then to review, your layer window should look something similar to this:
Notice how every major color area is separated and in its own layer. This will make it easier for us to shade each one. I also "locked" each layer transparency the same way I did for my inks layer.
Now, I create a new layer above my "Base color" layer, which really should read "Body" but never mind that. I name this layer "Shadows" but this time around I click on "Group with previous layer to create clipping mask" ("Group with previous layer" in older versions) option and change its mode to "Multiply". A clipping mask or grouping is a very useful set up. It basically means that you'll only be able to color inside the area covered by the layer under it. It will "clip off" anything extra. Try it! Pick a random color and try coloring ... let's say the hair. You won't be able to if its set up right. You will only be able to apply color to the same area that the body color covers. Your Shadow's layer should look something like this:
The little arrow means that it is a "clipping layer" or, in earlier versions, a "grouped layer". Why they change the terminology? I dunno, but they both mean the same. Oh, and if for some reason, you "unclip" or "ungroup" the layer, everything you colored in will be visible and it will look really messy. You can always fix that by going to Layer >> Create Clipping Mask or by pressing Alt + Ctrl + G.
Once you have your Shadows layer set up, its time to start shadowing! For this lineart, I wanted a "cell shading" type of look. So I choose my inks brush that I already set up and changed its hardness setting from 100% to 25%. This gave me a softer edge to work with.
Incidentally, I would recommend creating an "eraser" brush that has the same settings are your coloring brush. To do this, click on your eraser tool
(E) and choose "brush" from the drop down menu. The menu will change and look very similar to the regular brush menu. Then just change your settings as you did with your brush. Next, I decide where the light is coming from. In this case I decided that the light was coming from my left (the pony's right). Then I try to visualize where the shadows would fall on her body. I wasn't overly realistic with this particular piece, but I tried to keep it constant. It helps to observe how light reflects off different objects and textures. Eventually, you develop a knack for it. Or so I'm told.
You may have noticed that I turned off all the other color layers visibility. This is just to demonstrate shadows better. You don't need to do that. Ok! It feels like I been explaining this forever, so let's get going.
Pick a color that is lighter than your base color. Why lighter? because your layer is in multiply mode, which means that any color you use on it will appear darker on the layer below.
So in this case for my base color
I choose this color 
Giving me this:
Of course, you can choose any color you like. Try diferent shades and see what you get. Shading colors can affect the mood of a drawing greatly. For example, if you're drawing a scene that takes place at night, you might want to make your shadows purples and dark desaturated blues as opposed to black. Well, most shadows have a transparency to them, so if you're using black, you're blocking out your colors, but if you use purple or blue, your darkening your colors. It all depends on what you're looking for. In this case, I want happy neutral shadows! So I kept the color pretty much in the same range as the base color.
Moving on! Its time for secondary shadows!
Create a new layer the same way you did for the "Shadows" layer and call it "Shadows 2" or "Shadows II" or "Shadows The Revenge" or "Shadow layer that Cardinal made me do even though I think it looks just fine this way" Whatever you like but make sure its options and mode are the same.
I use secondary shadows to separate areas that overlap each other and to emphasize areas futher away from the light source. I picked a more redish pink
and again, I'm keeping this very simple and cartoony, so I place my secondary shadows sparingly. Like so...
I separated the hair from the body a little bit like so:
Furthest away from the light:
I repeat this process with every color layer. This my end result:
A note: Because the hair colors were close to each other or neutral (in the case of white), I kept them all in the same layer. However, in the case of very different colors like yellow and pink or rainbow hair, you may want to keep each color area in separate layers. It will make shading them a LOT easier.
We're almost done! Take a break! We'll be adding lights next!
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