<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
.sq{
width: 100px;
height: 100px;
border: 1px solid grey;
display:inline-block;
margin:0 10px;
}
</style>
</head>
<body>
<!-- © j0hnik //-->
<div class="sq">Элемент 1</div>
<div class="sq">Элемент 2</div>
<div class="sq">Элемент 3</div>
<div>
<input type="button" value="Какая-то кнопка" id="some_button"/>
</div>
</body>
<script type="text/javascript">
var colors = [
'#f00',
'#0f0',
'#00f'
];
document.getElementById('some_button').onclick=function(){
[].forEach.call(document.getElementsByClassName('sq'),function(node){
node.style.backgroundColor = colors[ Math.floor(Math.random() * colors.length)];
});
};
</script>
</html>