How to make block title words themable in D6
Quick and dirty in your block.tpl.php:

&lt;?php
  if ($block-&gt;subject=='Sign up for mail') {
    $block-&gt;subject='&lt;span class="red"&gt;Sign up&lt;/span&gt; &lt;span class="blue"&gt;for mail&lt;/span&gt;';
  }
?&gt;


And in your .css file:

.blue {
   color: blue;
}

.red {
    color: red;
}