Even if you are able to get the "perfect" color matching paint in the can, if it is a metallic paint, the spraying technique can radically affect the actual color on the car.
On the other hand, if you learn how to change the color with spraying technique, you can adjust the color to match.
I believe spraying a metallic paint wet tends to make the color lighter, because the metal flakes settle, and lay flat, reflecting more light.. Spraying dry tends to make the color darker, because the metal flakes stay on edge, and reflect less light.
Get a second bumper if you can. First, for practice, and second, your car will have a bumper while you are working on your car.