As a workaround you can style ASP:Labels with RadFormDecorator like that:
   1) Set a particular CssClass for all the ASP:Labels on the page. It can also be set by iterating through controls on the server-side.
   2) Use the RadFormDecorator CSS classes to create the desired styles for the particular skin. You can find an example below for the Metro and Glow skins:
CSS:
<style>
    .RadForm.RadForm_Metro .decorateLabel {
        background-color: #25a0da;
        color: #ffffff;
    }
 
    .RadForm.RadForm_Glow .decorateLabel {
        background-color: #ffa915;
        color: #ffffff;
    }
</style>
ASPX:
<form id="form1" runat="server">
    <telerik:RadScriptManager ID="RadScriptManager1" runat="server"></telerik:RadScriptManager>
    <telerik:RadSkinManager ID="RadSkinManager1" runat="server" ShowChooser="true" Skin="Metro"></telerik:RadSkinManager>
    <telerik:RadFormDecorator ID="RadFormDecorator1" runat="server" DecoratedControls="All" />
    <br />
    <br />
    <asp:Label ID="Label1" Text="this is label 1" runat="server" CssClass="decorateLabel" />
    <br />
    <br />
    <asp:Label ID="Label2" Text="this is label 2" runat="server" CssClass="decorateLabel" />
</form>

The above code renders like that - http://screencast.com/t/ggyQk9jlZ