There are different styles for shading a picture, but I generally follow the same procedure. First, I go back and select large areas and start shading with the airbrush set to a low pressure (about 10 or so). You can choose to shade in a layer above the colors so mistakes can be more easily recovered, but I usually color on top of the base colors in the same layer, so less memory is used and everything moves along faster. You may want to create larger brushes for easier shading, as the default brushes are all quite small. I like to at least have a brush with radius 100 and hardness 0.1.


For highlights, I like to use an airbrush with the mode set to Divide (Dodge) and a dark shade of gray. This makes the color look less flat, and more reflective. In PhotoShop, Color Dodge or Linear Dodge with white works well.