Not sure I completely understand but on the assumption that in the Library window you are viewing in Loupe mode a single image and also have the thumbnails on view normally with grey surround.
Would this work for you?
Select all your series of John and set the Colour label to red
On the Filter bar click on the red box to limit your thumbnail display to those red label images only
Now all the thumbnails will be shown with a pale red border replacing the usual grey but the one you are currently viewing will have a light grey surround (with a narrow red frame around the image) which should make it obvious which image you are viewing.
There must be an easier/quicker way but I do not know it
EDIT: added image of how I have been doing this I hope that by changing the thumbnail colour that the difference may be more pronounced?