Shading your drawing
Create a new layer for shading and name it what else? shading. Now you want your shading to appear above you color layer but under your outline layer. Move your shading layer between the other two layers. Now use the paintbrush tool and set your colors and add your shading along the edges and wherever you need it. When you are setting your shading colors experiment with the alpha slider to get different effects. Less alpha gives a lighter effect. This will look like a spray paint effect.
Highlights can be added just like shadows to give a little more 3d look. Don't put in too many highlights. Just a few here and there. Usually high points (reflected light) are not that prominent or large unless you are trying to simulate really bright lighting.
