Change the default border color of TextFormField in FLUTTER

Use enabledBorder of the InputDecoration, don’t forget you can also use the focusedBorder, like this :

InputDecoration(
                labelText: "Enter Email",
                fillColor: Colors.white,
                focusedBorder: OutlineInputBorder(
                  borderRadius: BorderRadius.circular(25.0),
                  borderSide: BorderSide(
                    color: Colors.blue,
                  ),
                ),
                enabledBorder: OutlineInputBorder(
                  borderRadius: BorderRadius.circular(25.0),
                  borderSide: BorderSide(
                    color: Colors.red,
                    width: 2.0,
                  ),
                ),
)

Here you have more info: https://api.flutter.dev/flutter/material/InputDecoration/enabledBorder.html