Can't change SimplePolylineGeometry to PolylineGeometry

CesiumJS
1

This bit of code works:

      const rLine = new Cesium.GeometryInstance({
      geometry: new Cesium.SimplePolylineGeometry({
        positions: Cesium.Cartesian3.fromDegreesArrayHeights(posns),
        colors: colorVals,
        colorsPerVertex: true, }),
      });

      viewer.scene.primitives.add(new Cesium.Primitive({
        geometryInstances: rLine,
        appearance: new Cesium.PerInstanceColorAppearance({
          flat: true,  }), 
     }));

But I need to make it a PolylineGeometry because SimplePolylineGeometry doesn’t support setting the width. If I make one change to
geometry: new Cesium.PolylineGeometry({
then my line won’t appear, no console errors.

How can I make this work so I can set a line width? I take it setting width for SimplePolylineGeometry is impossible?

Also tried this which gives me just a white line at the correct width:

 var polylines = viewer.scene.primitives.add(new Cesium.PolylineCollection());
    var polyline = polylines.add({
      positions : Cesium.Cartesian3.fromDegreesArrayHeights(posns),
      colors: colorVals,
      colorsPerVertex: true,
      width: 5
    });

I see now this is because Polyline has no mutli colour functionality but PolylineGeometry does! Not sure I Understand why there are two such similar sounding classes.