Display differently the intersection of two shapes

I have a 2d circle and a 2d rectangle, and I would like to display differently (different color or at least a line around the resulting shape), their intersection when they overlap.
What would be the simplest way to do this in Unity?

Assign a shader with additive/multiplicative blending to the material assigned to the shapes and the area of intersection will be correspondingly blended.

You could use the Silhouette Outline Shader from the Unity wiki:

http://wiki.unity3d.com/index.php?title=Silhouette-Outlined_Diffuse

Also check out my answer in this thread for a, in my opinion, better result: