Contractors apply several layers of paint to achieve a suitable finish. They leave the first coat to dry for the recommended time and apply one or more finishing coats. They might choose paints with special qualities for different types of room. Manufacturers have developed paints for bathrooms or kitchens that have good resistance to moisture for example. When they have completed painting, they clear away any equipment and restore the area to its original condition.
I agree with JHs. If there is no new color on it at all, just talk to the painter about it, if it was an honest mistake (which can happen very easily while painting) your painter should have no problem fixing it. However, in my years of experience, it is not unusual for 2 coats of door paint (good quality) not to cover very well at all. I once painted a red door 7 times, plus a tinted prime coat before I found the door to be a solid color.