obj.style.backgroundColor只能赋值，不能取值，取值要用getComputedStyle或者currentStyle。看下例函数：

function getRealStyle(id, styleName){
	var element = document.getElementById(id);
	var realStyle = null;
	if(element.currentStyle){
		realStyle = element.currentStyle[styleName];
	}else if(window.getComputedStyle){
		realStyle = window.getComputedStyle(element, null)[styleName];
	}
	return realStyle;
}