How to change the text color of the button theme in Flutter

If you use ButtonTextTheme.primary Flutter will automatically select the right color for you.

For example, if you make the buttonColor dark like this

  ThemeData(
    . . . 
    buttonTheme: ButtonThemeData(
      buttonColor: Colors.deepPurple,     //  <-- dark color
      textTheme: ButtonTextTheme.primary, //  <-- this auto selects the right color
    )
  ),

The text is automatically light. And if you make the buttonColor light, then the text is dark.

  ThemeData(
    . . . 
    buttonTheme: ButtonThemeData(
      buttonColor: Colors.yellow,         //  <-- light color
      textTheme: ButtonTextTheme.primary, //  <-- dark text for light background
    )
  ),