Add Shader (Fabric?) to Entity

CesiumJS
3

I have no idea what I'm doing wrong here

var f= viewer.entities.add({
    name: ...,
    id: i,
    position: new Cesium.SampledPositionProperty(),
    orientation: new Cesium.SampledProperty(Cesium.Quaternion),
    model: {
        uri: ...,
        scale: 1,
        minimumPixelSize: 42,
        defer: false,
    },
});
  f.model.material= new Cesium.Material({
  fabric : {
    type : 'Color',
    uniforms : {
      color : new Cesium.Color(1.0, 0.0, 0.0, 0.5)
    }
  }
  });
  f.model.material.uniforms.color = Cesium.Color.WHITE;
}

It loads the textured GLTF fine, but shouldn't it be replacing the material?