Plasterboard question

No, a 'plasterboard wall' would always have a skim of plaster over the plasterboard paper, so it's not the same. The bare surfaces do take paint well, except it's not very strong as you just have a layer of paper/card over the gypsum, so are prone to taking damage.

The finished coat of plaster skim would be different to the internal surface of the board itself (not to mention, you're not taking off that paper, it's literally bonded and cooked to the plaster itself).

Once plasterboard is installed, it's then plastered over with a skim coat, left to dry, then you still need to paint on several mist coats to provide a clean finish for wall paint. You're not going to get any more of an accurate colour trying to do what you're proposing than by just getting sample paints and putting them on a wall, and eventually painting over them once you've picked your colour.