HTML5 Tree Navigation - Accessible Markup with ARIA attributes for screen-readers

Markup

<nav>
    <ul role="tree" tabindex="0" aria-labelledby="label_1">
        <li role="treeitem" tabindex="-1" aria-expanded="true"><a href="#">Colours</a></li>
        <li role="group">
            <ul>
                <li role="treeitem" tabindex="-1"><a href="#">Red</a></li>
                <li role="treeitem" tabindex="-1"><a href="#">Blue</a></li>
                <li role="treeitem" tabindex="-1"><a href="#">Yellow</a></li>
                <li role="treeitem" tabindex="-1"><a href="#">Green</a></li>
                <li role="treeitem" tabindex="-1"><a href="#">Orange</a></li>
            </ul>
        </li>
    </ul>
</nav>