My white 06 is usually so full of gravel chips, bugs, and road stains, that I can't tell what color it is. 500 miles a week will do that. I had the front bumper replaced and painted by a non Toyota body shop at 1200 miles after nailing a truck tire tread. I didn't notice any mismatch colors either before or after. Its not real apparant to me from your picture, but I'd tend to just live with it. Unless its a concorse show car, the road rash will take over pretty soon, anyway. But that's just me.
I think from what I see in the photos, and I would have a body shop confirm this, I don't think the rear bumper has been clear coated which means it probably been repainted after it left the factory.
Another thing that could change the final color is not enough base coat. The plastic that the bumper is made out of is black and it take quiet a bit of base coat to completely cover black with white. If someone totally replace the bumper with a new one and painted it off the car that would be very easy to do.
Call me overly critical but I finally got tired of looking at the my poorly matched bumpers. On my 15K checkup I showed it to the service manager and they understood why it was bothering me. Without any hemming or hawing or excuse making, they set me up with a loaner for tomorrow and will pull the bumpers and repaint them attempting to get a closer match to the body color. Over the past year I've seen superwhite camry's and corolla's without issues and tons of white prius's with this mismatch problem to lesser and rarely sometimes greater degrees. I've seen the problem on silver's and driftwood too. Hard to believe a car like prius from a company like toyota would settle for such poor finish results. Even ford and chevy does a better job color matching paint on so it looks like the same color on both plastic and metal surfaces. I'll let you know how it turns out.