There is currently no property of the RadTileViewItem control that fascilitates easily chaging the header background. We will add such a property as soon as our schedule allows us to, hopefully for the upcoming service pack.
So, at the moment the only way to cutomize the header background is to edit the control template. The easiest way to customize the header at runtime would be to have a style (in XAML) specifying the modified control template in the resources of your page/application, get a reference to that resource, and set the style to the TileViewItem at the moment you need to.
You may find the following resources useful:
I hope I was able to help. Please let me know if you have any other questions.
All the best,
the Telerik team