How do I use RGB color in Flutter?

You can also use the hexadecimal representation Color(0XFF212845).
Comment from source

/// In other words, if AA is the alpha value in hex, RR the red value in hex,
/// GG the green value in hex, and BB the blue value in hex, a color can be
/// expressed as const Color(0xAARRGGBB).

I am using this code block for a personal project of mine in order to show text with a specific color using Color.fromRGBO, first parameter is Red, second is Green, third is Blue and last parameter defines the Opacity.

Text(
    "This is a sample text",
     textAlign: TextAlign.center,
     style: TextStyle(
             color: Color.fromRGBO(255, 179, 102, 1)
    )
)

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(),
                ))