Android - Problema de Color en background en Android Studio

<resources>

    <string name="app_name">Temperature Converter</string>

    <string name="myColor">#F5F5F5</string>

    <string name="celsius">to_Celsius</string>

    <string name="fahrenheit">to_fahrenheit</string>

    <string name="hello_world">HelloWorld!</string>

    <string name="menu_setting">Setting</string>

    <string name="title_activity_main">TemperatureConverter</string>

    <string name="calc">Calculate</string>

</resources>

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:app="http://schemas.android.com/apk/res-auto"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:background="@string/myColor"

    tools:context="com.example.smb22.temperatureconverter.MainActivity">

    ...