Secure $_GET

Hello,

I have the following code in my WP:

<?php $bg = $_GET['q']; if ($bg == black) {?> output <?php } else {} ?>

So, if I put in URL somesite.com?q=black, then I will get the output. It works.

My question is about potential security issues with this method. Is it safe or should I filter or sanitize the variables and how do I do so.

Thanks.