Use el color RGB en Flutter
Color.fromRGBO(38, 38, 38, 0.4)
Proud Polecat
Color.fromRGBO(38, 38, 38, 0.4)
Color.fromARGB(0xFF, 0x42, 0xA5, 0xF5);
Color.fromARGB(255, 66, 165, 245);