Eye bases come in a variety of colors. For instance, if you apply a dark base, like black, in to the outer corners and apply a white base on to your lid. Make sure to blend it out, but not too much or you'll end up blending out the product too much Blend out the edges. Now, apply a white shadow on top of the base on the lid, blending out the ends. For the dark base, apply a dark shadow to the outer corners on top of the base. In this example, by applying these bases under the shadows you are giving the shadow something to stick to and the best part about using a base is that it can really bring out and make your shadows pop!
Now you can play around with the different bases, use a variety of colors to see what does what.
Check some bases out here http://www.misssandycandy.com/primers--bases.html