How to change textField underline color?

I'm new to both flutter & dart. Currently, using this in one of my personal projects.

In all of my form, the underline of textField is showing in blue color. I want to change that to some other color. The piece of code which I'm using is like...

new TextField(
  controller: this._emailController,
  decoration: new InputDecoration(
      hintText: "Enter your email",
      labelText: "Email",
      labelStyle: new TextStyle(
          color: const Color(0xFF424242)
      )
  ),

),

Can't able to understand how to achieve this.

Note: I know there is a similar question at here https://stackoverflow.com/questions/48706884/change-textfields-underline-in-flutter. But, at there also it has not completely solved. Also, one more link which looks similar to mine at here https://stackoverflow.com/questions/26574328/changing-edittext-bottom-line-color-with-appcompat-v7 but, that actually belong to Android development by using JAVA not DART(flutter) which I'm using for my android app development. So, please don't get confused about those links.

Just used -:

decoration: InputDecoration(        
              enabledBorder: UnderlineInputBorder(      
                      borderSide: BorderSide(color: Colors.cyan),   
                      ),  
              focusedBorder: UnderlineInputBorder(
                      borderSide: BorderSide(color: Colors.cyan),
                   ),  
             ),

it works for me :)

** See update below or see the answer by @GJJ2019 **

The logical answer would be to use an InputBorder, particularly an UnderlineInputDecorator, and pass it in to the inputdecorator as the border. However, all this does is tell the InputDecorator whether is should use an underline or whatever else you specify.

The actual color is based on the theme - from the source:

Color _getActiveColor(ThemeData themeData) {
  if (isFocused) {
    switch (themeData.brightness) {
      case Brightness.dark:
        return themeData.accentColor;
      case Brightness.light:
        return themeData.primaryColor;
    }
  }
  return themeData.hintColor;
}

So to change the colour do something like this (or specify the theme for your entire application):

new Theme(
  data: new ThemeData(
    primaryColor: Colors.red,
    accentColor: Colors.orange,
    hintColor: Colors.green
  ),
  child: new TextField(
    decoration: new InputDecoration(
      hintText: "Enter your email",
      labelText: "Email",
      labelStyle: new TextStyle(color: const Color(0xFF424242)),
      border: new UnderlineInputBorder(
        borderSide: new BorderSide(
          color: Colors.red
        )
      )
    ),
  ),
),

UPDATE:

This is now possible to do in the way you'd expect it to work.

decoration: InputDecoration(        
  enabledBorder: UnderlineInputBorder(      
    borderSide: BorderSide(color: theColor),   
  ),  
  focusedBorder: UnderlineInputBorder(
    borderSide: BorderSide(color: theColor),
  ),
  border: UnderlineInputBorder(
    borderSide: BorderSide(color: theColor),
  ),
)

To change the color for the entire app, use ThemeData's inputDecorationTheme property.

• To use the color only when the input is in focus (i.e. clicked & ready to type):

  MaterialApp(
    ...
    theme: ThemeData(
      inputDecorationTheme: InputDecorationTheme(
        focusedBorder: UnderlineInputBorder(
          borderSide: BorderSide(color: Colors.red)
        ),
      )
    )
  )
  

• To always use the color (even when not in focus), set enabledBorder and border as well:

  MaterialApp(
    ...
    theme: ThemeData(
      inputDecorationTheme: InputDecorationTheme(
        focusedBorder: UnderlineInputBorder(
          borderSide: BorderSide(color: Colors.red)
        ),
        enabledBorder: UnderlineInputBorder(
          borderSide: BorderSide(color: Colors.red),
        ),
        border: UnderlineInputBorder(
          borderSide: BorderSide(color: Colors.red),
        ),
      )
    )
  )
  

    decoration: new InputDecoration(
              labelText: "Email",
              suffixIcon: Icon(Icons.email),
              labelStyle: TextStyle(color: Colors.red),
              enabledBorder: new UnderlineInputBorder(
                  borderSide: new BorderSide(color: Colors.red)
              )
        )

need change three border.

  enabledBorder: OutlineInputBorder(
            borderSide: BorderSide(color: _type.color),
          ),
  focusedBorder: OutlineInputBorder(
        borderSide: BorderSide(color: _type.color),
  ),
  border:
    OutlineInputBorder(borderSide: BorderSide(color: _type.color)),

By using InputDecoration, you can set underline color as per your requirement.

TextField(
            decoration: InputDecoration(
              hintText: "Enter your email",
              // [enabledBorder], displayed when [TextField, InputDecoration.enabled] is true
              enabledBorder: UnderlineInputBorder(
                borderSide: BorderSide(color: Colors.lightBlueAccent),
              ),
              //[focusedBorder], displayed when [TextField, InputDecorator.isFocused] is true
              focusedBorder: UnderlineInputBorder(
                borderSide: BorderSide(color: Colors.redAccent),
              ),
            

) )

with focusedBorder property in TextField can change underline color like : focusedBorder: new UnderlineInputBorder( borderSide: new BorderSide(color: Colors.black), ),

@override
  ThemeData appBarTheme(BuildContext context) {
    return Theme.of(context).copyWith(
        primaryColor: Colors.transparent,
        appBarTheme: AppBarTheme(elevation: 0),
        inputDecorationTheme: InputDecorationTheme(
            border: UnderlineInputBorder(
                borderSide: BorderSide(style: BorderStyle.none, width: 0))));
  }