How to access a color in code behind which is defined in a xaml file

It try to change a ForeColor or Backcolor with a color which is defined in another seperated xaml file. If i use this:

_toolTip.ForeColor = (Color)Application.Current.Resources["Color_001"];

I will get a exception thrown at the start of the programm: System.InvalidCastException: "Specified cast is not valid."

If i use this:

_toolTip.ForeColor = Resources["Color_001"] as Color;

It tells me that the class Color does not allow Null and cant be referenced with as-operater.

Is there a way i missed out?

Code in ColorBrush.xaml

<Color x:Key="Color_001">#FFFFFFFF</Color>

Code in App.xaml

<Application.Resources>
    <ResourceDictionary >
        <ResourceDictionary.MergedDictionaries >
            <ResourceDictionary Source="pack://application:,,,/ColorBrush.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</Application.Resources>

Code in App.xaml.cs

        private void DesignToolTip(string tooltipText)
    {
        _toolTip = new System.Windows.Forms.ToolTip();
        _toolTip.OwnerDraw = true;
        _toolTip.UseAnimation = true;
        _toolTip.Draw += GetToolTipDraw;
        _toolTip.Popup += GetToolTipPopUp;
        _toolTip.ToolTipTitle = tooltipText;
        _toolTip.AutoPopDelay = 500;
        _toolTip.AutomaticDelay = 500;
        _toolTip.ForeColor = (Color)Application.Current.Resources["Color_001"];
        //_toolTip.ForeColor = Resources["Color_001"] as Color;
        //_toolTip.ForeColor = System.Drawing.ColorTranslator.FromHtml("#FFFFFFFF");
        _toolTip.BackColor = System.Drawing.ColorTranslator.FromHtml("#FF181818");
    }