The canvas of the endpoint was a DOM-element. I was working with it.
<html>
<head>
<script type="text/javascript" src="js/jquery.js"></script>
<script type="text/javascript" src="js/jquery.jsPlumb-1.2.6-all.js"></script>
</head>
<body>
<div id="Node"></div>
<script>
jsPlumb.addEndpoint("Node", {uuid: "id_my"});
$("#id_my").css({"background-color":"blue", "opacity":0.3, "height":"100px"});
</script>
</body>
</html>