My car needs a new rear bumper

I can both order a primered bumper or a factory painted one, of course the latter costs almost double, and takes 5 weeks, but I would presume it would be a nearer / safer color match of my existing bumper than to have my new bumper painted by a repair shop?

I'm confused, some say factory painted might not Match at all and that it's better to paint I locally......

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10 740Li
11 Mini John Cooper Works
12 Range Rover Evoque Dynamic Si4