Obtain:
// get by ID
var aa = document.getElementById ('bo') ；
//Get and return a collection according to the element name
var bb = document.getElementsByTagName('input');
Change the background color:
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>Switch background color</title>
<script>
function changeColor(color)
{
var obt1 = document.getElementById('bo');
//obt1.bgColor= 'blue';
obt1.bgColor= color;
}
</script>
</head>
<body bgcolor="red" id="bo" >
<input type="button" onClick="changeColor('yellow')">
</body>
</html>