Need ideas: Assign distinct color to each face of mesh

He took the world normal and turned it to a color vector information straight ahead I suppose, combining it with the world position he did the same thing with as well.

If you want a specific color per triangle, it is more difficult, since this color attribute would belong to a vertex, and face is composed of 3 verticies, shared or not. It'd have to be the same color for all three verticies so that it does not interpolate to different color.

So I started with dot(CameraVector, WorldNormal) in vertex shader:

Maybe I can make some progress based on this.

Not sure if this helps, but if you want a color per face, then all vertices per face should be unique and assigned that color (so no vertex re-using).