You can change the color of the underline of a TextField in Flutter by using the border property of the InputDecoration class. You can set the border property to UnderlineInputBorder, and set the borderSide property to a BorderSide with the desired color.
Here's an example:
TextField(
decoration: InputDecoration(
labelText: 'Username',
labelStyle: TextStyle(color: Colors.grey),
enabledBorder: UnderlineInputBorder(
borderSide: BorderSide(color: Colors.red),
),
focusedBorder: UnderlineInputBorder(
borderSide: BorderSide(color: Colors.blue),
),
),
)
In this example, we're setting the enabledBorder to a red BorderSide, and the focusedBorder to a blue BorderSide. You can customize the colors as per your requirement.