The real problem for paint shade differences, especially in bumper fascias, is that the fascias are supplied and painted by a separate vendor. . . not GM and shipped to the assembly plants. Meanwhile, the body and associated panels, hood, etc are done in the assembly line paint shop. You can thank outsourcing for that.I had a Silver TL and it had the same shade shift on the bumpers. In all fairness, this is a common issue across many cars due to the flex additives in the paint in the bumpers. It happens to show up really bad on white or silver cars.
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Now, with computer paint code matching, the colors are supposed to be able to be matched exactly. But as many know, especially with certain colors and metallic paints, the shading can vary significantly between paint batches and mixes and even angle of application by the sprayer or type of sprayer used.
If the bumper fascias were painted in house like they used to be, the same paint would be applied as on the body and color and shading mismatches would be much less likely.