Radio Button Unselected Color in Flutter

Set unselectedWidgetColor prop of theme in MaterialApp like below:

return new MaterialApp(
      title: 'xyz',
      home: xyz(),
      theme: ThemeData(
        brightness: Brightness.dark,
        unselectedWidgetColor:Colors.white
      ),
    );

A useful tip that read the source code of radio.dart and you will get everything!

Sure, you have to change the ThemeData of the container of your Radio buttons.

Assuming that you are using a Row Container:

  Theme(
        data: ThemeData.dark(), //set the dark theme or write your own theme
                  child: Row(
          children: <Widget>[
            Radio(
              //your attributes here...
            ),
            Radio(
              //your attributes here...
            )
          ],
        ),
      )

How it works?

Because if you check the code of the ThemeData, you'll see this validation

unselectedWidgetColor ??= isDark ? Colors.white70 : Colors.black54;

then dark theme is using white70

In case you want only to change Border Color for Widget when it's unselected

Just Customize Fill Color property

Radio(
            //here -----
            fillColor:  
            MaterialStateProperty.resolveWith<Color>(
                  (Set<MaterialState> states) {
                return (isSelected)
                    ? CustomColors.green
                    : CustomColors.greyMedium;
              }),
            // ----

              value: 'en',
              groupValue: langValue,
              activeColor: Theme.of(context).primaryColor,
              onChanged: (value) {},
            ),

Wrap your Radio Widgets with Theme widget and add theme data with unselectedWidgetColor as color you want

Ex:

              Theme(
                  data: ThemeData(
                    //here change to your color 
                    unselectedWidgetColor: Colors.White,
                  ),
                  child: Row(
                    children: <Widget>[
                           Radio(), 
                           Radio()
                    ],
                  ),
                );