I experienced a similar problem when I got the sideskirts for my 7th gen painted. My painter sprayed them with 040 super white to match the rest of the car, but there were three different shades of 040 on his database. They all use the same name and paint code, but they have slightly different tints to them. The only way to get it 100% accurate is to do tests with all 3 of them against the OEM paint.
Fading is a factor too. The OEM paint would have most definitely faded a little bit over the years, so even if you had the perfect match it may still be a tiny bit off if you have really sharp eyes. The paint on the actual metal body of the car fades the fastest, whereas the plastic bumpers tend to hold their original colour better (or at least this was the case on my 7th gen). The engine bay, trunk compartment, door jambs etc will all remain perfect, for obvious reasons. I wouldn't recommend painting the entire car one panel at a time on different days, but at least you'd know the colour is going to be the same all round by doing it that way