I don't know of any existing plugin, but this should be possible in a custom theme, or a layer plugin.
Override a function such as post_meta_who, then check the user type against the constants from qa-include/app/users.php, and change the colour accordingly.
See http://docs.question2answer.org for more details on writing plugins.
Here's an example overriding the post_meta_who function:
public function post_meta_who($post, $class)
{
$class = '';
switch ($post['raw']['level']) {
case QA_USER_LEVEL_SUPER:
case QA_USER_LEVEL_ADMIN:
$class = 'usertype-admin';
break;
}
$post['who']['data'] = '<span class="' . $class . '">' . $post['who']['data'] . '</span>';
parent::post_meta_who($post, $class);
}
You'll need to expand the different cases in the switch for each user type (see the constants defined at the top of qa-include/app/users.php).