How to modify the comment body while in comment.tpl.php
If you insist, you can always go like that:

 print str_replace(
   array('♦','♥'),
   array('&lt;font color="red"&gt;♦&lt;/font&gt;', '&lt;font color="red"&gt;♥&lt;/font&gt;'),
   render($content)
 );


But instead of deprecated &lt;font&gt;, use &lt;span class=""&gt; and CSS. Consider classes for all 4 colours, for black and red, and for card marks altogether, for future styling ease.