the different of IE and Chrome when return the color style

<style>
div{
width:50px;
height:50px;
}
</style>
<script>
function showColor(obj){
alert(obj.style.backgroundColor);
}
</script>
<div id='a' style='background-color:red;' onclick='showColor(this)'></div>
<div id='b' style='background-color:#eeeeff' onclick='showColor(this)'></div>
<div id='c' style='background-color:rgb(1,11,111)' onclick='showColor(this)'></div>