This off white you are seeing on slabs or pre hung doors is the primer. This process will save you or the builder one step in the finishing process.
In my region, in a new home, these doors are all set up in the garage and spray painted and then hung when dry. the frame is all installed and they just pull the pins and assembly line spray them. then do the simple re hang. You on the other hand may not have a spray rig and can just take the door off and lay it on a pair of horses and paint one side then wait till dry and do the flip side. Then the easier way is to paint them hung in their respective spot.
Now, being a door and taking all the traffic, semi gloss is the paint sheen of choice, one main reason is that its cleanable and scrubable. Flat paint is not, if you do the paint will marr and even get dirtier. Semi gloss paint is smooth and non porus, flat is full of microscopic hills and valleys that only collect dirt and grime.
So, another consideration is to paint this door the sheen of all the other doors in your home.
I hope that I got around and to your initial question.