break it down like this:
Diffuse Pass -self shadowing and accurate light sources of final concept. Its just a grey scale render.
Spec/Reflection pass -all the shiny bits
Color Pass- just the flat color maps without any shadow or light
Shadow Pass.
At the Diffuse Pass, you just need paint in grey scale and get the composition and lighting correct.
Multiply a Color layer on top of this to give the scene the basic texture it needs.
Use linear dodge layer for all Spec and Reflections for shiny parts
And shadow pass.
Its not the traditional way for paint, but its very similar. Its a commmon technique to do a grey scale painting, and then add tints of color on top and the final stage is the specular and shadows.
The method teaches you to break it down to the basics at the diffuse stage without worrying about the tedious details.
