To change this, the default control template of the RadPane element will have to be extracted and modified. More specifically, locate the MultiTrigger for when the IsPinned and IsSelected properties are True and modify the Setter for the Background property of the Border element with x:Name="BorderVisual". After that, create a new Style with TargetType="RadPane" and set the modified template to the Template, LeftTemplate, TopTemplate, RightTemplate, and BottomTemplate properties.
With this being said, I have attached a sample project for you to test.