Pass an extra vector from a material to a post-processing material

Hello all!

I am trying to make a material that switches between two gradients depending on lighting conditions. So, I need to pass at least two colors per material to a post-processing material. The first color can go through the diffuse SceneTexture easily enough, but how can I pass along the second color? I tried to use the SpecularColor scene texture, but the actual specular node on the material is single channel.

Will I have to write a new shading model to be able to pass specular color properly?