The splash color of the button's InkWell.
The ink splash indicates that the button has been touched. It appears on top of the button's child and spreads in an expanding circle beginning where the touch occurred.
The default splash color is the current theme's splash color, ThemeData.splashColor.
The appearance of the splash can be configured with the theme's splash factory, ThemeData.splashFactory.
final Color? splashColor;