Look at this sample image of a pressbook page: http://www.emovieposter.com/images/announcements/coolitems/pb_big_cage_backcover.jpg
Yes, you COULD restore one of these one-sheets from the pressbook image, but you sure would be doing a lot of guessing on color and fine detail.
And WHO did the recreation of the missing part (or the restoration of the other posters)? And if no other example of this poster exists, how did that restorer know what to draw, and what colors to use?
Well if it had been restored using that pressbook there might be a tiger up in the corner.
WHO Cares who did the restoration? (aside from the fact it looks like their business would boom is people knew).
But it's not like they took a b&w poster, gave it to a restorer and said color it in. Obviously the image is very similar to the WC, which you, me, and everyone else has seen - so the artist had a good idea of what he/she had to recreate.
From the description, it seems that there is a good indication that the colors were known also from what was left of the poster. Could it be possible that the top right border was really blue and didn't match the lower rust-colored border at all? I guess so. Maybe in the original Harlow had a few more individual eyelashes drawn. Who cares? You are comparing it to an item that is not known to exist at this time.
The way the poster presents now I am sure is infinitely more appealing than how it presented before. If I was the proud purchaser of this poster, I would be perfectly fine with the guesses as to color and fine detail the restorer made.
You obviously aren't pleased and so I imagine will not be bidding on it. That's cool too. You can wait for the next one.