If you assign it to a class it should work:
<script>
function changeClass(){
document.getElementById('myButton').className = 'formatForButton';
}
</script>
<style>
.formatForButton {
background-color:pink;
}
</style>
<body>
<input id='myButton' type=button class=none value='Change Color to pink' onclick='changeClass()'>
</body>