There should be no difference in color between the bumper and body panels. Those are all painted in house. You may notice a very slight variation between say a side mirror and a body panel due to the painted side mirror being supplied by a vendor. Lexus looks at paint match under varying indoor and outdoor lighting conditions, both artificial and natural. I think it may be, as explained in another post, the fact that the bumper has a small amount of "flex" to it that may give an impression of improper paint match.