<!DOCTYPE html>
<html>
<head>
<title>Sayfa başlığı</title>
</head>
<body>
<div id="kutu" style="width:300px; height:100px; background:red">
<button onclick="renkDegistir()"> renk değiştir </button>
</div>
<script>
function renkDegistir(){
document.getElementById('kutu').style.background="green";
}
</script>
</body>
</html>