Tempest,
<!doctype html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Title</title>
<style type="text/css">
div{
width: 100px;
height: 100px;
border: 1px dashed Gray;
padding: 5px;
}
</style>
</head>
<body>
<div></div>
<div id="box1"></div>
<div id="box2"></div>
<script>
var box = document.getElementsByTagName("DIV");
for (var i = 0; i < box.length; i++)
box[i].onclick = function() {
if (this.style.backgroundColor == "") this.style.backgroundColor = "green";
else this.style.backgroundColor = ""
};
</script>
</body>
</html>