Develop little Javascript to notify Iframe after successful upload in Wordpress

Hi. I need to let users of my wordpress site upload a file. This is implemented by using a plugin and works correctly. But after successfully uploading the file, I want to change the content of an iframe on the same page. The iframe page itself also works - it takes a parameter in the url and then displays what i want. The only thing is letting the iframe page know to update after the file upload was successful. So I need a Javascript code of about 15 to 20 lines. It is about the following page:[login to view URL] This very early stage website. On this page in the lower part you see yellow area. This is an iframe with [login to view URL] behind. And you see an upload button below. What I want is that after upload (*.png or *.jpg) the color of the iframe changes (to a random colore). Usage of color php is simple [login to view URL] for example. It is just for testing. Can you help me on that? Please check and let me know. Thanks and kind regards