In WinForms, it goes something like this:
richTextBox.SelectionColor = Color.Black;
richTextBox.Appendtext("OrderConfirmation id is: 88899 Price : 222 ");
richTextBox.SelectionColor = Color.Blue;
richTextBox.AppendText("BUY ");
richTextBox.SelectionColor = Color.Black;
richTextBox.AppendText("Symbol: ");
richTextBox.SelectionColor = Color.Red;
richTextBox.AppendText("ABB ");
richTextBox.SelectionColor = Color.Black;
richTextBox.AppendText("and Date :201-01-01");
If you're doing the psuedo richtext in Silverlight or WPF, you can easily find out how to do it using google.