Hi Bob,
Thank you for contacting us.
It depends on what backColor is serialized in design-time. You can take a look at the Form's Designer.cs file and search for ContentPanel's BackColor property. If you would like you can manually change the backColor in the constructor of your application's form after the call of the InitializeComponent method. There you can set the ContentPanel's BackColor property without any problems.
Please write me back if I could be of further help.
All the best,
Boyko Markov
the Telerik team