In silverlight the Controls have ControlTemplates which can be edited. Foreground, BorderBrush and Background are properties of the Control class but if we want to expose properties for all elements in RadMenu we will need MouseOver-Background/OuterBorder/InnerBorder, TickIcon-Fill/Stroke, Separator line-1/2 etc. The right way to edit these colors or the elements in the RadMenu and RadMenuItem is to edit their templates. Further more in the RadMenuItem we have a ButtonChrome that we use to render the Highlighted state of the RadMenuItem.
You can check the attached file if you are interested how to edit the RadMenu in Blend you can check the project attaced in this forum post
the Telerik team