flutter 自定义颜色

flutter有四种自定义颜色的方法：如下
Color c = const Color(0xFF42A5F5);//16进制的ARGB
Color c = const Color.fromARGB(0xFF, 0x42, 0xA5, 0xF5);
Color c = const Color.fromARGB(255, 66, 165, 245);
Color c = const Color.fromRGBO(66, 165, 245, 1.0);//opacity：不透明度