让下拉框中的每项有不同颜色的背景

用HTML版上的控件，并把此DropDownList1转化成服务器控件使用.
定制的方式：
   DropDownList1.Items.Add(new ListItem("默认","white"));
   DropDownList1.Items.Add(new ListItem("红色","red"));
   DropDownList1.Items.Add(new ListItem("兰色","blue"));
   DropDownList1.Items.Add(new ListItem("紫色","SandyBrown"));
   DropDownList1.Items.Add(new ListItem("青色","Aqua"));
   DropDownList1.Items.Add(new ListItem("黄色","Yellow"));
   for (int i= 0 ;i < DropDownList1.Items.Count;i++)
   {
    DropDownList1.Items[i].Attributes.Add("style", "background-color:" + DropDownList1.Items[i].Value);
   }