<!DOCTYPE html>
<html>
<head>
<script language="javascript" type="text/javascript">
var button = document.getElementById("txt");
var color = document.getElementById("word").style.color
function changeColor(color) {
color = "blue";
};
button.onclick = changeColor();
</script>
</head>
<body>
<h1 id="word">Hello world</h1>
<button id="txt" onclick="changeColor()">Click here</button>
</body>
</html>