You know, I've noticed that a few of the Toyotas in my area have a non-matching rear bumper. They are new ones, still on the dealer lot. They are always silver or red and they are mostly Camrys, Solaras, or Corollas.
I asked my Dad why the bumpers were like that. (He worked for Ford as an engineer in a plant in Michingan that formed and painted bumpers) He told me that happened all the time. He said that sometimes, when a bumper is painted, the paint doesn't come out just right. They basically re -prime and repaint it. Many times when they repaint it, it comes out perfect, but sometimes it doesn't. The problem is, you can't tell the bumper isn't exactly the same color until it is on the car. At that point it's too late. You could get Toyota to put a new bumper on your car ( Dad has had this done on a Ford or two ), but it is not 100% that it will match either. I say unless it really bugs you, to live with it and not go through trying to find a matching bumper