How to get color of iframe in php webpage?

I want to create color picker which will select color from other websites onclick in PHP web page.

I have given option to add url for reference website for color selection in my webpage. And I am opening reference url in Iframe but I am not getting color of iframe on clicking of it.

<!DOCTYPE html>
<html>
    <head>

        <script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js"></script>
        <script>
          $('iframe').load(function(){   $($("iframe")[0].contentWindow).on('click', function(event) {  alert('test'); });});
        </script>
    </head>
    <body>
        <iframe id="iframe" src="http://www.theuselessweb.com/" width="500px;" height="500px">
        </iframe>
        <br>
    </body>
</html>

Solution

I got alternate option to get colors from other website. To get color first I am getting screenshot of entered url using google pagespeed api and from that image I am getting colors codes