The color for the radio's Material.
Resolves in the following states:
If null, then the value of activeColor with alpha kRadialReactionAlpha, focusColor and hoverColor is used in the pressed, focused and hovered state. If that is also null, the value of RadioThemeData.overlayColor is used. If that is also null, then the value of ColorScheme.secondary with alpha kRadialReactionAlpha, ThemeData.focusColor and ThemeData.hoverColor is used in the pressed, focused and hovered state.
final MaterialStateProperty<Color?>? overlayColor;