I've heard that too. However, in practice bumpers and other exterior painted trim are not generally not painted at the same as the car body (or even in the same factory, on the same paint line or with the same paint). Sometimes even the paint supplier is different! The only time when this is different would be on special vehicles where the painting is sub-contracted to a supplier to do the whole thing althouh this is not usually done on volume production cars like the 147, it's more likely on prestige or niche vehicles where the cost is not an issue.
The important thing with colour match for metallics and pearlescents (and therefore to much the same extent with Irredescent paints like Nuvola) is that colour match is only a problem on ajoining surfaces if they run together at the same angle. e.g if there is no change in surface contour between bumper and wing (156 bumpers spring to mind). The more the bumpers are designed to blend in with the form of the car body, the more obvious a difference in colour match will be.
If there is a change in contour between the parts, then the light reflects differently and the colour match doesn't matter so much (this is not always the case for a solid colour though).
With a spoiler there is generally a distinct change in angle of the ajoining surface or a larger gap between body and spoiler to lessen the impact of a colour difference.
There is bound to be some difference in the colour of the spoiler because the paint will be a different batch and the production process is differrnt too but unless the whole car is painted at once this can't be avoided. Also, unless I'm mistaken, the largest surface area (and therefore the one where any colour difference is going to be most obvious) will be on the top of the spoiler. So unless you are looking at the roof this won't be particularly obvious!
I wouldn't worry about it.
Alternatively, you could always get the whole thing re-sprayed!