[bes]

можно и как-то так

<style>
.but1:hover {
	background: green;
}
.but2:hover {
	background: gray;
}
</style>
<button class="but1">click</button>
<script>
window.onload = function () {
	document.getElementsByTagName('button')[0].onclick = function () {
		if (this.className == 'but1') {
			this.className = 'but2';
		} else {
			this.className = 'but1';
		}
	}
}
</script>