js时间定时器实现屏幕变色

<script type="text/javascript">
			var a=["red","orange","yelllow","green","cyan","blue","purple"];
			var num=0;
			function b(){
			document.getElementsByClassName("d1")[0].style.background=a[num];
			num++;
				if(num>7){
					num=0;
				}
			}
			setInterval(function(){b()},500);

</script>