That's not fading, that's just a poor match. Any paint can be matched, even if it's faded. Have a look around the hinges for colour difference, overspray and signs of disturbance. I would guess that it's been painted, even a replacement door in colour wouldn't be as bad a match as that.
At the end of the day, cars get bumped and scraped, and it's not uncommon to find repairs. If they are done correctly, there's nothing wrong with that, but that looks very poor and will give you problems when you come to sell the car on or trade it in.
To be fair, if it's a decent job, I would rather see a repainted door than repairs around a chassis leg or boot floor, for example.