<RichTextBox x:Name="OutputText"  Margin="10,128,10,10" Grid.RowSpan="2" Grid.ColumnSpan="2" ScrollViewer.CanContentScroll="False" 
VerticalScrollBarVisibility="Visible" HorizontalScrollBarVisibility="Visible">           
        </RichTextBox>

 var st = cmb.SelectedItem.ToString() +
                @"0";
            for (int ir = 0; ir < 5; ir++)
            {
                st += ir.ToString();
                for (int iС = 0; iС < 16; iС++)
                {
                    st += iС.ToString();
                    st += "some long text very long text ";
                }
                st += "\n";
            }
            OutputText.PutBeutifulText(st+st+st + st + st);

  public static void PutBeutifulText (this RichTextBox OutputText, string text)
        {
            if (!String.IsNullOrEmpty(text) && text.Length>0)
            {
                

                var markerText = (определяется переменная для цвета текста);

                OutputText.SelectAll();
                OutputText.Selection.Text = "";

     var tb = new TextBlock();
                tb.TextWrapping = TextWrapping.NoWrap;
                tb.FontFamily = new FontFamily("Courier New");//("Century Gothic");
                tb.Foreground = markerText.GetBrush();
                tb.Text = text;

                Paragraph p = new Paragraph();
                p.Inlines.Add(tb);
                OutputText.Document.Blocks.Add(p);
                OutputText.HorizontalScrollBarVisibility = ScrollBarVisibility.Visible;

		OutputText.ScrollToEnd();
            }
        }