I don't know what the deal is with bumpers, and do not claim to be an expert painter, but will say what I have seen.
Bumper covers rarely match the vehicles. When I was an estimator at a Chevy dealership I used to point out to the customers the color differences on the new vehicles that were parked in a line on the lot. That definately helped with any problems about color match on bumper jobs.
When I was an insurance adjuster there was a shop that always wanted something more on every job. On one job they told the customer that they could not guarantee the color match on a bumper job because the insurance company would not pay for blending the front of the vehicle. The customer called me all upset that her vehicle was not going to be right and it was because of me. I had pointed out the color difference when I wrote the estimate, but they had forgotten about that. I had seen several cases like that where the bumper matched when painted, but within a couple of months it was a different color. Since that customer was a claimant, and the shop was a thorne in my side, I approved the blending. I also told the customer that it was not common practice to blend onto the rest of the car, but to make her feel more comfortable I would approve it. I also told her that any problems with color match at a later time was between her and the shop. A couple of months later the customer called to tell me that she was having the bumper repainted again as it doesn't match anymore.
I have also seen cases where the bumper was painted at the same time as the whole car. The bumpers again did not match after a few months.
I will suggest one thing that I have thought of, and someone can correct me on this. Many people still feel that they have to use a "flex agent" when painting bumper covers. This "flex agent", when added, will reduce the paint mixture to some extent. I understand that the "flex agent" only stays in the paint for a relatively short period. Is it possible that when the "flex agent" leaves the paint, it is actually changing the color?
I am only posting this as an observation from my experience, and not to cause any type of arguements.
Aaron