title: WPF: reverting brush to default/original
body: <p>I'm a complete newbie at WPF. </p>
  
  <p>At the moment I'm making a usercontrol for form elements called "LabeledTextbox" which contains a label, a textbox and a textblock for errormessages.</p>
  
  <p>When the using code adds an errormessage, I want to put the border of the textbox in red. But, when the errormessage gets removed, I'd like to turn back to the default bordercolor of the textbox. 
  I feel there must be a very easy way to do this.</p>
  
  <p>My code:</p>
  
  <p><em>(in public partial class LabeledTextbox : UserControl)</em></p>
  
  <pre><code>public string ErrorMessage
  {
      set
      {
          if (string.IsNullOrEmpty(value))
          {
              _textbox.BorderBrush = Brushes.Black; //How do I revert to the original color in the most elegant way?
          }
          else
          {
              _textbox.BorderBrush = Brushes.Red;
          }
  
          _errorMessage.Text = value;
      }
  }
  </code></pre>