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
    )
  ),

I believe the more updated answer is mainly found here: https://flutter.dev/docs/release/breaking-changes/buttons

elevatedButtonTheme: ElevatedButtonThemeData(
  style: ButtonStyle(
    backgroundColor: MaterialStateProperty.all<Color>(Colors.black,), //button color
    foregroundColor: MaterialStateProperty.all<Color>(Color(0xffffffff),), //text (and icon)
  ),
),

Depending on the button change...

elevatedButtonTheme: ElevatedButtonThemeData()
outlinedButtonTheme: OutlinedButtonThemeData()
textButtonTheme: textButtonThemeData()

Suragch's answer is correct, but sometimes we want to set a completely custom color as button text color. It is achievable by providing a custom colorScheme with secondary color set:

buttonTheme: ButtonThemeData(
  buttonColor: Color(0xffff914d), // Background color (orange in my case).
  textTheme: ButtonTextTheme.accent,
  colorScheme:
    Theme.of(context).colorScheme.copyWith(secondary: Colors.white), // Text color
),

Related: As I stumbled upon this when looking specifically for TextButton color, setting the MaterialApp theme solved that:

theme: ThemeData(
      textButtonTheme: TextButtonThemeData(
        style: TextButton.styleFrom(
          primary: Colors.blueGrey[900],
        ),
      ),
    ),

found on https://www.woolha.com/tutorials/flutter-using-textbutton-widget-examples