Floating action button: how to change the splash color and highlight color like in a RaisedButton?

If you check the source code of FloatingActionButton the splash color is tied to the Theme of the app, so you can change the theme only for the button, like this:

Theme(
  data: Theme.of(context).copyWith(highlightColor: Colors.yellow),
  child: FloatingActionButton(
    backgroundColor: Colors.red,
    onPressed: () {},
    child: Text("hello world"),
 ),
)

While the current answer works for setting the highlight color of a single FloatingActionButton, you could also define the splash color and highlight color globally. The splash color can be defined directly for the FloatingActionButton, though, the highlight color comes from your theme.

final theme = ThemeData(
  highlightColor: Colors.white.withOpacity(0.25),
  ...,
  floatingActionButtonTheme: FloatingActionButtonThemeData(
    splashColor: Colors.white.withOpacity(0.25),
    ...,
  ),
);

Then, simply add your theme to your MaterialApp like this

  MaterialApp(
    theme: theme,
    ...,
  )