material style change placeholder color code example

Example 1: css placeholder color

::-webkit-input-placeholder { /* Chrome/Opera/Safari */
  color: pink;
}
::-moz-placeholder { /* Firefox 19+ */
  color: pink;
}
:-ms-input-placeholder { /* IE 10+ */
  color: pink;
}
:-moz-placeholder { /* Firefox 18- */
  color: pink;
}

Example 2: text field placeholder color flutter theme

ThemeData( 
  // (...)
  hintStyle: TextStyle(
     color: Colors.grey, // <-- Change this
     fontSize: null,
     fontWeight: FontWeight.w400,
     fontStyle: FontStyle.normal,
     ),
);