Are you talking about the separator that appears while dragging items?
If this is the case, then you have to edit the Template of RadTreeView. In the Template there is a resource with the following key:
|<SolidColorBrush x:Key="DragBetweenItemsFeedback_BackgroundBrush" Color="#FFFA8C3B"/>
Changing the color of this SolidColorBrush will change the color of the separator.
If you have any additional questions, let us know.
the Telerik team