How to remove background color of TextField on Flutter Widget?

You should set filled to true.

TextField(decoration: InputDecoration( fillColor: Colors.red, filled: true)),

Wrap your TextFormField inside a Container and change its color property to match your background color (as from your picture I'll assume its white):

     Container(
        color: Colors.white, // or any color that matches your background
        child: TextFormField(
                 decoration: InputDecoration.collapsed(),
                 validator: (input) => input == "" ? 'The task is empty' : null,
                 onSaved: (input) => task = input,
               )
     ),