I looked a bit at this issue separately the other day. What I observed: low grade books (<1 for sure, not sure about the cutoff) with something missing get a blue label that notes incomplete, higher grade books get a better grade but a green label. NG seems reserved for books without any cover, or a reproduction cover. I'm not positive about missing only the back cover. I didn't find any examples, but I'd guess blue with notes b/c the back cover is like a missing centerfold or other book parts often denoted as missing. At this low a grade, IDK if it matters what color the label is. The value would either be personal attachment or being an affordable way to complete a collection when the price of the book is otherwise prohibitive.