How do modify color on an WEB page Element, using APPBuilder

I have a label called OutputAreaDis (code below) depending on the on the Status I want to modify the  background color.
How do I modify  the color  document.getElementById returns Null.  I can I get access to the element to change the color?

Thank you

if (xmlhttp.readyState==4 && xmlhttp.status==200)

  {

  //var results = document.getElementById("OutputAreaDis");  <===  Returns a NULL

  //results.innerHTML = "Success";

  OutputValue = Output.setText("Success");

  }

  else if (xmlhttp.status != 0)

     OutputValue = Output.setText("Failed"+" STATUS="+xmlhttp.status+" "+xmlhttp.statusText);