2D模式下加载GeoJSON文件颜色比3D模式的要暗很多，怎样使2D模式的颜色也变的比较亮？

如题，2D模式下加载GeoJSON文件的数据颜色比3D模式的要暗很多，怎样使2D模式的颜色也变的比较亮？ 源码：

<script>
    var viewer = new Cesium.Viewer('cesiumContainer', {
        sceneMode: 2
    });
    var promise = Cesium.GeoJsonDataSource.load('grid222.geojson');
    viewer.dataSources.add(promise);
    viewer.flyTo(promise);
</script>

JSON文件如下： {

"type" : "FeatureCollection",	
"features" : [{
		"type" : "Feature",
		"geometry" : {
			"coordinates" : [[[114.37999106868926, 36.0854368313549], [114.37999106868926, 36.087285894830124], [114.38214958987339, 36.087285894830124], [114.38214958987339, 36.0854368313549], [114.37999106868926, 36.0854368313549]]],
			"type" : "Polygon"
		},
		"properties" : {	               
			"fill" : "#FF0000",
			"fill-opacity" : 1,
            "stroke-width" : 1,
			"stroke" : "#FF0000",
			"stroke-opacity" : 1                   
		}
	}		
]

}