<asp:Repeater ID="rpH" runat="server" OnItemDataBound="rpH_ItemDataBound" >
    <ItemTemplate>
        <div class="h_element">
            <asp:Panel ID="pnl" ClientIDMode="Static"  runat="server">
               <h4> <%# DataBinder.Eval(Container.DataItem, "ArticleName") %> </h4>
               <%# DataBinder.Eval(Container.DataItem, "ArticleText")%> </asp:Panel>
        </div>
    </ItemTemplate>
</asp:Repeater>

protected void rpH_ItemDataBound(object sender, RepeaterItemEventArgs e)
    {
        
        if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
        {
            Panel pnl = (Panel)e.Item.FindControl("pnl");
            if (pnl != null)
            {
                string[] hclasses = { "hbRed", "hbGreen", "hbBlue" };
                
                Random rnd = new Random();
                pnl.ID = "pnl" + e.Item.ItemIndex;
                pnl.CssClass = hclasses[rnd.Next(2)].ToString();
                
            }
        }
    }

........
<div class="h_element">
<div id="pnl0" class="hbRed">
......
</div>

</div>
<div class="h_element">
<div id="pnl1" class="hbRed">
.......
</div>

<div class="h_element">
<div id="pnl2" class="hbRed">
.......
</div>
.........

Random rnd = new Random();