make listbox items colorful

protected void Page_PreRender(object sender, EventArgs e)
{
    bool flag=false;
    foreach (ListItem li in ListBox1.Items)
    {
        if (flag)
        {
            li.Attributes.Add("class", "optred");
            flag = false;
        }
        else
        {
            li.Attributes.Add("class", "optblue");
            flag = true;
        }
    }
}
<style type="text/css">
.optred{background-color:red;}
.optblue{background-color:blue;} 
</style>