특정 li에 addClass 될 경우 경고창 띄우기가 가능한가요?

///html

<div class="main sec03">
    <h2>Media</h2>
    <ul class="cboth">
        <li style="background-color:red" class="on"><span></span></li>
        <li style="background-color:green"><span></span></li>
        <li style="background-color:blue"><span></span>
            <video class="lhmb__bg" src="http://www.carepilates.com/main03.mp4" data-m-src="http://www.carepilates.com/main03.mp4" data-w-src="http://www.carepilates.com/main03.mp4" loop="" muted="" playsinline="" id="video3">
                            </video>
        </li>
    </ul>
</div>

 

//script
$(".sec03 li").click(function(){
        $(this).addClass("on");
        $(this).siblings().removeClass("on");
    });