Edit in JSFiddle

var $items = $('#list').children(),
    $betas = $('.beta');

// removes $betas from $items
$items = $items.not( $betas );

$items.addClass('hilite');

<ul id="list">
    <li class="alpha">Alpha</li>
    <li class="alpha">Alpha</li>
    <li class="beta">Beta</li>
    <li class="beta">Beta</li>
    <li class="alpha">Alpha</li>
    <li class="beta">Beta</li>
    <li class="alpha">Alpha</li>
    <li class="beta">Beta</li>
</ul>

.beta { color: red; }
.hilite { background: yellow }