I was taking another look at the WPF expander control that I talked about here and discovered that you don't actually need to use any code to get rid of the background colour of the control when collapsed.
What I discovered is that if you set the VerticalAlignment of the expander control. You can set this property to Bottom, Center, Stretch and Top. All but Stretch will hide the background colour of the control when the control is collapsed.
Here is the revised XAML markup for the control:
