Before you even start actually shading, you need to look at your scene and decide where the shadows should go. Just like looking at the overall scene for the backgrounds, you now need to make sure you're noting where the light source is coming from and what things in the scene could be block ing it. After you have everything worked out, it's time to shade.

Cel shading is quite often simply labeled "anime" shading by those who see it, just because anime is so common nowadays. Really though, many types of animations use it. Really it doesn't matter what you think of it as. It's technical term is cel-shading, and that's what we're going to go with.

To start shading your characters, you're going to need to start by creating another new layer. This layer should be above the colors layer and below the lines layer. You should also change the blending mode to multiply and the opacity of the layer to about 60%.

Multiply will cause a "blending" effect with the layers below it when you apply colors.

The layer will be have a lighter opacity because it allows for more blending between the two layers. You can opt to use darker colors to shade and change the opacity to compensate, but 60% is just what I work at.

From here you will begin painting shadows with the brush tool. You should lower the hardness a bit, to maybe about 80-90%. This will put a slight softness to the shadows so they don't feel too pasted on.

Colors for shading
You want to paint with colors appropriate to the lighting. The basic colors you'll use are a deep brown with a hint of red in it and a deep mellow blue. The brownish redworks on most skin colors and a lot of deep colors like blues and reds. The mellow blue is great for whites and light colors. You have to experiment as you go though.

You may want to change the colors you shade with to similate types of light. For instance you can add a lot more red or orange to a color for sunsets which would be casting a deeper glow.

After that, it's pretty much the basic shading and finishing up. The only thing you may consider is multiple layers of shading. You can stack shading to put deep shadows. Simply do as you did before and create another layer above the current shading layer set to "multiply".

Highlights
Highlights are pretty much the same as doing shadows but in reverse. You're going to create a new layer above the shadows layer. Actually, I put it above the lines layer, which will allow it to draw over the lines, but I think it helps with highlights. This is one of the times that preference comes out again though. You want to set your layer's blending property to "Screen".

Now it's pretty much as simple as selecting the color you plan to highlight. For example, you can do highlights in hair by grabbing the actual color of the hair and painting on the screen layer. It should come across like a sharp color highlight.

The same method applies for the rest of the piece. If you think a color needs to be brighter, simply use a slightly brighter color. Consider using different colors to create tones like once again for sunsets. Other than that, if you don't like the way something looks, you can always play around with the settings and paint with a low opacity which can make subtle colors. Find your style.

By now if you've read my previous sections, you probably already know what this is if you didn't already. This is for mixing in elements of painting, doing realistic gradual shades instead of a flat color for shading.

It can be done the same way as before, using a new layer set to multiply for its blending mode. You can just leave the overall layer opacity to 100% and instead grab a brush set to 0% hardness and with a very low opacity like around 5%. You can paint the colors gradually this way by going over the same areas several times.

The other option is setting up your brush to paint more opacity based on pen pressure. This way you can paint do a very quick gradual paint easily.

Now that you have learned to figure out the scene and paint with the light source, you need to figure out when to break that rule. This is a rule that applies mostly only to comics, especially manga/anime inspired ones.

Basically, the concept is that light can set a mood in a scene. You can use these techniques every once and awhile to create an effect rather than paint reality. It's just not something to become abused or it will look fake instead.