How to change TextFormField input text color in Flutter

Doing UI for a flutter app at uni, I just want the text typed into the TextFormField to be white. It seems unnecessarily difficult. I've tried googling etc but can't see an obvious answer.

  new Theme(
    // this colors the underline
    data: theme.copyWith(
      primaryColor: Colors.white,
      hintColor: Colors.transparent,

    ),
    child: new Padding(
      padding: const EdgeInsets.fromLTRB(32.0, 40.0, 32.0, 4.0),
      child: TextFormField(

          key: Key('username'),
          keyboardType: TextInputType.text,
          controller: usernameController,
          decoration: InputDecoration(

              fillColor: Colors.black.withOpacity(0.6),
              filled: true,
              border: new OutlineInputBorder(

                borderRadius: const BorderRadius.all(

                  const Radius.circular(8.0),
                ),
                borderSide: new BorderSide(
                  color: Colors.transparent,
                  width: 1.0,
                ),
              ),
              labelText: 'Username',
              labelStyle:
                  new TextStyle(color: Colors.white, fontSize: 16.0)),
          style:
              TextStyle(fontSize: 20.0, color: textTheme.button.color),
          validator: validateUserName,
          onSaved: (val) => this.loginFields._username = val),
    ),
  ),

This will do:

TextFormField(
  style: TextStyle(color: Colors.white),
)

Puedes usar style: TextStyle

body: Center(
    child: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: <Widget>[
            Form(
                key: _formKey,
                child: Column(
                    crossAxisAlignment: CrossAxisAlignment.center,
                    children: <Widget>[
                        TextFormField(
                            TextFormField(
                                controller: field,
                                style: TextStyle(fontSize: 18, color: Colors.red),
                                decoration: const InputDecoration(
                                    contentPadding: const EdgeInsets.only(
                                        left: 15,
                                        top: 8,
                                        right: 15,
                                        bottom: 0
                                    ),
                                    hintText: 'name',
                                ),
                                validator: (value) {
                                    if (value.isEmpty) {
                                        return 'Please enter some text';
                                    }
                                    return null;
                                },
                            ),
                        )
                    ]
                )
            )
        ]
    )
)