我想在flutter中更改聚焦的otp文本字段的单个元素的颜色

TextField buildOtpTextField() {
  return TextField(
    keyboardType: TextInputType.number,
    maxLength: 4,
    obscureText: true,
    decoration: InputDecoration(
      counterText: '',
      contentPadding: EdgeInsets.symmetric(vertical: 10.0),
      border: OutlineInputBorder(
        borderRadius: BorderRadius.circular(10.0),
      ),
      focusedBorder: OutlineInputBorder(
        borderSide: BorderSide(color: Colors.blue),
        borderRadius: BorderRadius.circular(10.0),
      ),
    ),
    onChanged: (value) {
      // 处理输入值改变的逻辑
    },
  );
}

@override
Widget build(BuildContext context) {
  return Scaffold(
    appBar: AppBar(
      title: Text('OTP输入'),
    ),
    body: Center(
      child: Padding(
        padding: EdgeInsets.all(20.0),
        child: buildOtpTextField(),
      ),
    ),
  );
}