<ul>
<?php $i = 0; ?>
<?php foreach ($comments as $comment) : ?>
<?php $i++; ?>
<li id="comment-<?php comment_ID() ?>"<?php if($i&1) { echo 'class="odd"';} else {echo 'class="even"';} ?>>

.odd {
background-color: #fcf9fc; }
.even {
background-color: #616161; }