Like in my other setups the Material Slot has NO use until you apply the modifier (except the Stage Material). The Materials are configured with the "set material Node" in the Geometry Node Setup and not with the material slot. If you work with new materials, you have to change in the "Material" Box in the Geometry Node Material Switch! (or after applying the Modifier).

In this Blend File you´ll find some special materials

1. Random Color Node
2. Gradient Node with a Spherical setup