At a Motor factors, the paint would be mixed from scratch, they can mix any shade, that they have a formulation for... whoever mixes the paint will check the colour of the car, with the relevant colour charts, and make a decision to pic the closest shade, always get them to match the paint to the area of car that is to be painted, as the rest of the car may be off colour!
Most decent painters, would always pic the No1 shade, that is usually the light shade to re spray a car, and not necessarily the shade that left the factory! this is for ease, incase the car was damaged at a later date, and returned for repairs, the painter could establish the new colour with ease, rather than guess work..
Thom