APPBuilder: How do modify color on an WEB page Element with JS?

How do you modify the background color of a WEB Element in this case a Label?

How can you access the WEB Element, getElementByid  returns Null??
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);