Terrain OGL Viewport Colors

[galagast]

I'm trying to understand how the Height Field Quick Shade is shading the terrain in the viewport. Somehow, modifying and or re-creating a simplified version is proving difficult.

Currently, in order to learn it, I'm simply trying to create a viewport display that has an option to just set a Base Color, and a Slope Color.

The Base Color is just an overall color for the whole terrain. The OGL Diffuse seems to work on this case (channel referenced in this case).
The Slope Color is masked by a custom Slope field (generated by a HF Mask by Feature).

Internally, I'm just using a Color Mix node with the Slope Field set to the bias like so:

The setup works as expected when rendered:

And by using the HF Quick Shade (as seen in the viewport):

 

But by trying to re-create a simpler setup.. I'm unable to properly show the colors that I want in the viewport:

How do I set the OGL Diffuse to the output of say, a Color Mix VOP?

Here is my current network and H16 file:

terrain_vp_colors.zip