The primary color of the button when the button is in the down (pressed) state. The splash is represented as a circular overlay that appears above the highlightColor overlay. The splash overlay has a center point that matches the hit point of the user touch event. The splash overlay will expand to fill the button area if the touch is held for long enough time. If the splash color has transparency then the highlight and button color will show through.
Defaults to the Theme's splash color, ThemeData.splashColor.
final Color splashColor