How to change Text color depending on background so that there is good contrast between the two colors

@override
Widget build(BuildContext context) {
  var backgroundColor = Colors.orange; // this color could be anything

  return Scaffold(
    body: Center(
      child: Container(
        padding: const EdgeInsets.all(12),
        color: backgroundColor,
        child: Text("Hello World"), // if backgroundColor: Colors.orange this is visible but if backgroundColor: Colors.black, it isn't visible
      ),
    ),
  );
}

Output (backgroundColor: Colors.orange, Text is visible):

Output (backgroundColor: Colors.black, Text isn't visible):

you can use computeLuminance() of Color class. Something like this:

@override
Widget build(BuildContext context) {
  var backgroundColor = Colors.orange; // this color could be anything
  var foregroundColor = backgroundColor.computeLuminance() > 0.5 ? Colors.black : Colors.white; 

  return Scaffold(
    body: Center(
      child: Container(
        padding: const EdgeInsets.all(12),
        color: backgroundColor,
        child: Text("Hello World", style: TextStyle(color: foregroundColor)), 
      ),
    ),
  );
}

Of course you need to play around with the luminance IF-statement ... 1.0 is white and 0.0 is black. Yellow for instance is something about 0.8.