Edit in JSFiddle

var yellow = false;

function hallo() {
  //alert("Hallo Welt!");
  yellow = !yellow;
  var div = document.getElementById("content");

  if (yellow) {
    div.style.color = 'yellow';
    div.innerHTML = "Das ist gelber Text";
  }else{
     div.style.color = 'green';
    div.innerHTML = "Das ist grüner Text";
    
  }
}

<div id="content" onclick="hallo()">Hier Klicken!</div>

#content {
   font-family:arial,sans-serif;
  font-size:24pt;
  background-color: red;
  color: green;
  text-align: center;
  cursor: pointer;
}