I need label above billboard image. The label color is white, But it become dark. Here is code. disableDepthTestDistance seems necessary.

viewer.entities.add({
      position: Cesium.Cartesian3.fromDegrees(item.coordinates[0], item.coordinates[1]),
      show: true,
      label: {
        text:  'spring',
        fillColor: Cesium.Color.WHITE,
        showBackground: false,            
        style: Cesium.LabelStyle.FILL,     
        show: true,
        scale: 0.4,
        disableDepthTestDistance: Number.POSITIVE_INFINITY 
      },
      billboard: {
        image: mapLabelBg,
        width: 150,
        height: 150,
        scale: 0.5,
        disableDepthTestDistance: Number.POSITIVE_INFINITY,
       },
  });

You can see label spring is not pure white. it's dark. I don't add opacity to label. How can i make label 100% white?