This is a sample project which shows how to change the shadow color in the deferred lighting pipeline.
You can change the shadow color with "_ShadowManager" object.
This project uses a modified version of Internal-PrePassLighting.shader to support the shadow color. It runs only on Unity 4.2 because the shader is modified from [the built-in shader in 4.2] (http://unity3d.com/unity/download/archive). You have to manually modify Internal-PrePassLighting.shader [like this change] (https://github.com/keijiro/unity-deferred-shadow-color/commit/5a1b69560cf671ceff8346d4b761f39cbd5d445c) to run it on the other versions of Unity.