Thank you for writing.
When you set the BackColor to the control, as you can see it styles the whole control. To achieve your scenario, after setting the Shape of the PanelElement, you need to introduce your BackColor setting to the FillPrimitive. Here is an example:
radPanel1.PanelElement.PanelFill.GradientStyle = GradientStyles.Solid;
radPanel1.PanelElement.PanelFill.BackColor = Color.Red;
I hope that you find this information useful. Should you have any other questions, do not hesitate to contact us.
the Telerik team