I hadn't seen the show in color at the time, so I didn't have any other referents beyond what was on the box. Well, that and the cover image on The Making of Star Trek -- I think I had some doubts about which one to go with, but I ended up going with the box art for whatever reason. I have a vague recollection of discussing the choice with my father, maybe, but I don't remember which of us was advocating which option.
That sounds like my experience. I had The Making of Star Trek
, and later, The World of Star Trek
, but for some reason, I thought the box photo was "just right."
I think my mind changed once seeing larger color photos of the ship in the ST Poster Books a few years later. Why that convinced me when the paperback covers did not is mystery I cannot explain!