How to create a shader where white part is the same as background's colour & shadow is opaque?

Hello. I am new to blender. I want to try 2D & 3D combination. I’ve been trying to get a specific shader node. Here’s the image on what I have right now.

Left side is the shader I am trying to ‘fix’. Right side is very basic cell shade. Light is on the left side.

The drawing is what I am going for:

I want this effect to keep Blender’s 3D shading while also be able to draw over the object with grease pencil. I already have the shader I want with this monkey example.