javascript:
<script type="text/javascript">
function GetBackColor() {
document.getElementById("testButton").setAttribute("style", "background-color:green");
}
function RemoveColor() {
document.getElementById("testButton").removeAttribute("style");
}
</script>
button:
<input type="button" id="testButton" onmouseover="GetBackColor()" value="test" onmouseout="RemoveColor()"/>