About Treeview Nodes Backcolor

You'll need to do a couple things:

1. You'll need to track the "currentHighlightID" so you can change the backcolor back to white (or your default)
2. You'll need to have a click event handler on the treeview (which I'm guessing you already have

What you'll do in your click is check to see if currentHighlightID has a value and if so set the backcolor of that node to the default:
 
TreeView.Nodes[currentHighlightID].BackColor = defaultColor;
 
Then you can set the backcolor of the currently clicked upon node to your highlight color and save the ID of the current node to currentHighlightID.