Drupal 7 Comments Alternating Row Colors
Looks like you can use the $zebra class in the comment template.

Add it in the preprocess hook as a class:

/**
 * Implements hook_preprocess_comment
 */
function THEME_preprocess_comment(&amp;$variables) {
  // Zebra classes are added in the default template_preprocess function
  if (!empty($variables['zebra'])) {
    $variables['classes_array'][] = $variables['zebra'];
  }
}


You can then style the 'odd' and 'even' comments with CSS

e.g, something like:

.comment.odd{
  background-color: #efefef;
}
.comment.even{
  background-color: #ffffff;
}