改变Richtextbox中线条的颜色

第二行，换成另一种颜色

我的代码如下:

 private void WindowLoaded(object sender, RoutedEventArgs e)
    {
        UpdateRTBOnDifferentThread("stackoverflow.com" + Environment.NewLine+"stackoverflow.org" + Environment.NewLine);
    }
    private void UpdateRTBOnDifferentThread(string message)
    {
        Dispatcher.Invoke(DispatcherPriority.Normal, new DispatcherOperationCallback(delegate
        {
            var newExternalParagraph = new Paragraph(new Run(message)) {Foreground = Brushes.Black};
            richTextBox1.Document.Blocks.Add(newExternalParagraph);
            return null;
        }), null);
    } 

第二行是:"stackoverflow.org"

我想改变输入文本