Previously I discussed adding base colors and why they're a good tool to use when establishing a mood in an illustration. Now it's time to add depth to the image by adding shadows and highlights.
Step 4 : Shadows
I like to start with shadows first and then add the highlights after. Shadows can really set an image into a scene by pushing the foreground into the background. They can quite easily add volume and dimension to an image and help to give it form.
However, they can be quite tricky too. I try to never use solid black when I make illustrations as it tends to be a little too heavy. By adding a little color you can still get some interesting detail within the dark nooks of an image. Plus, black doesn't have any color information within it so if you're trying to establish a mood, adding black can break the effect.
Step 4 : Highlights
The highlights are where things really tend to pop. Once the shadows are in place, you can use highlights to pull elements of the image towards you and help to define the forms you're trying to illustrate. Highlights can also help to describe the texture of an object. smooth and shiny objects will tend to have very bright and tightly drawn highlights, while objects that are more dull will have very weak and diffused highlights.