Adobe Illustrator tutorial - The Gradient tool

Part 7: The gradient tool. So as you may have learned by now, Adobe Illustrator is a vector based drawing program so you're somewhat limited to solid colors. That is unless you are using the gradient tool! Gradients provide smooth transitions of color and I think they add much needed dimension and softness to flat vector drawings.

It's the closest your going to get to airbrush-like shading. The only downside is that you're limited to linear and radial transitions, that is unless you are using the gradient mesh tool (Not important for a beginner but can be utilized more once you become familiar with all the tools.)

A great link here that explains the gradient tool.