var entities = dataSource.entities.values; 
                console.log("entities_positions"); 
                for (var i = 0; i < entities.length; i++) { 
                    entity =  entities[i]; 
                      var cartesianPositions = entity.polyline.positions; 
                      var cartographicPositions = viewer.scene.globe.ellipsoid.cartesianArrayToCartographicArray(cartesianPositions._value); 
                      var longitude = cartographicPositions[0].longitude; 
                      var latitude = cartographicPositions[0].latitude; 
                      var height = cartographicPositions[0].height; 
                      console.log(longitude); 
                      console.log(latitude); 

                } 

Those coordinates are given in radians. You can convert them back to degrees with Cesium.Math.toDegrees(radianValue).

How can i toggle multiple labels/entities ?

var viewer = new Cesium.Viewer('cesiumContainer', { infoBox : false });
var entities = viewer.entities;

var labels = entities.add(new Cesium.Entity());

var longitude = - 106;
for (var i = 0; i < 5; ++i) {
    entities.add({
        parent : labels,
        position : Cesium.Cartesian3.fromDegrees(longitude, 45.0),
        label : {
            text : i.toString()
        }
    });
    longitude++;
}

viewer.zoomTo(viewer.entities);

Sandcastle.addToolbarButton('Toggle Labels', function(){
    labels.show = !labels.show;
});

How does segmenting a polyline into different colors work.

var viewer = new Cesium.Viewer('cesiumContainer');

var primitive = new Cesium.Primitive({
  geometryInstances : new Cesium.GeometryInstance({
    geometry : new Cesium.PolylineGeometry({
      positions : [
        Cesium.Cartesian3.fromElements(-2822918.76, -1347627.64, 5539750.33),
        Cesium.Cartesian3.fromElements(-2860091, -1790883.7, 6104936.09),
        Cesium.Cartesian3.fromElements(-2869994.31, -2212991.67, 6598178.01)
      ],
      width : 2.0,
      vertexFormat : Cesium.PolylineColorAppearance.VERTEX_FORMAT,
      colors: [Cesium.Color.RED, Cesium.Color.BLUE],
      colorsPerVertex: false
    })
  }),
  appearance : new Cesium.PolylineColorAppearance({
    translucent : false
  })
});

viewer.scene.primitives.add(primitive);