As any painter will tell you, prep work is just as important -- if not more important -- than the paint application itself. If the paint on your house is chipping or peeling, it needs to be scraped and sanded smooth before applying a new finish. In most cases, you won’t need to remove all the existing paint, but scraping, sanding, patching, caulking, and priming are critical steps that take place before the start of most exterior painting jobs.
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.