How do I use RGB color in Flutter?

Try using

Color.fromRGBO(38, 38, 38, 0.4)

Where r is for Red, g is for Green, b is for Blueand o is for opacity

Example:

Container(
                width: double.infinity,
                height: double.infinity,
                color: Color.fromRGBO(38, 38, 38, 0.4),
                child: Center(
                  child: CircularProgressIndicator(),
                ))