fillColor property

MaterialStateProperty<Color?>? fillColor
final

The color that fills the radio button, in all MaterialStates.

Resolves in the following states:

This example resolves the fillColor based on the current MaterialState of the Radio, providing a different Color when it is MaterialState.disabled.
link 
Radio<int>(
  value: 1,
  groupValue: 1,
  onChanged: (_){},
  fillColor: MaterialStateProperty.resolveWith<Color>((Set<MaterialState> states) {
    if (states.contains(MaterialState.disabled)) {
      return Colors.orange.withOpacity(.32);
    }
    return Colors.orange;
  })
)

If null, then the value of activeColor is used in the selected state. If that is also null, then the value of RadioThemeData.fillColor is used. If that is also null and ThemeData.useMaterial3 is false, then ThemeData.disabledColor is used in the disabled state, ColorScheme.secondary is used in the selected state, and ThemeData.unselectedWidgetColor is used in the default state; if ThemeData.useMaterial3 is true, then ColorScheme.onSurface is used in the disabled state, ColorScheme.primary is used in the selected state and ColorScheme.onSurfaceVariant is used in the default state.

Implementation

final MaterialStateProperty<Color?>? fillColor;