Main Window - Tree | Indicate Thumbnails                    Ctrl+Shift+I

Folder Popup Menu - Options | Indicate Thumbnails

 

 

Checking this option causes ThumbsPlus to use differently colored folders to indicate "interesting" folders. Folders for folders which contain thumbnails are colored green; folders for folders which do not themselves have thumbnails, but whose subordinate folders do, are colored blue. This option is really useful only after drive(s) have been scanned for images.

 

See also: Options | Customize - Colors and Thumbnail | Scan system.