Contour is a contour line (edge detection) filter for Unity.
Download one of the unitypackage files from the Releases page and import it to the project.
Add the Contour component to a camera. Done.
Not only the colors but also opacities of contour lines and background can be changed by setting alpha channel values of Line Color and Background Color.
Contour supports three types of sources -- Color, Depth and Normal. The Normal source is only available in the deferred shading path.
In the typical usage, the Depth source is used to add silhouette lines to objects, but it's not good at detecting boundaries between contacting objects (e.g. feet and ground). The Normal source is useful for such cases.