dynamically change the color of text as requested by user asp.net c#

//Store orginal text in hdnSearchText.Value (HiddenField)

 public string HighlightText(string argText)
 {
     string Search_Str = hdnSearchText.Value.ToString();
     // Setup the regular expression and add the Or operator.
     Regex RegExp = new Regex(Search_Str.Replace(" ", "|").Trim(), RegexOptions.IgnoreCase);
     // Highlight keywords by calling the
     //delegate each time a keyword is found.
     return RegExp.Replace(argText, new MatchEvaluator(ReplaceKeyWords));
 }

 public string ReplaceKeyWords(Match m)
 {
   return ("" + m.Value + "");
 }

 //Search Button Click Code
 protected void btnSearch_Click(object sender,EventArgs e)
 {
  string arg = txtSearch.Text; // Get Search Term
  string result = HighlightText(arg);
  lblDisplay.Text = result; //Display result in any label or html editor 
 }

StringBuilder builder = new StringBuilder();
          builder.Append("adf");
          builder.Append("<span style=\"color:Blue;\">");
          builder.Append("[text to highlight]");
          builder.Append("</span>");
          lbl.Text = builder.ToString();