The term that describes the apperant mismatched panels is called Metamorism. Color is merely the reflection of light. So different light sources provide a different shade of color just as mentioned on this thread. Its not a ' mismatch' or a 'flop' difference, merely the reflection of light off of two different angles of panels! Its perfectly illustrated in the GM service bulleton with the painted panel with the seam which changes colors when the adjacent panel angle is changed.
As for the bumper facias and fuel door being different colors is a mismatch which is better on some makes and models than others. Being a production painter for 25 years trust me when i tell you acura bumpers are by far the worst. Check them out next time you see one and youll be glad that GMs quality standards are better than Acura! The mismatch happens for several reasons. One is that the plastic parts follow a different refinishing procedure because they have extra steps to insure adhesion to the raw plastic. So they are painted in a different part of the factory or a different facility entirely. Both pose color match challenges. The biggest one is the fact that paint is pumped sometimes miles to different parts of the factory to the spraying robots. The longer the distance the more likely the heavier toners/metallics/pearls in the color settle to the bottom of the lines, effectively changing the original color that matched when it started at the pump! Another reason there is a slight color and flop mismatch especially on metallic colors is the fact that the metallics and micas lay different on plastic panels because plastic by nature collects more static electricity.
Its very diffecult to achieve a very good color match throughout entire car even when the car is sprayed my the same painter with the same gun/mix/pressure and technique! Ferrari is by far the best color match but there completly built/painted by hand by skilled technicians. $$$
Just my 2c to help clarify. BTW i have a summit white 2011 SLP585 and the front bumper matchs because they respray the front clip to ensure the hood matchs. Rear bumper and fuel door color is off but with what i know about the challenges the factory has with color match, im perfectly happy with the slight mismatch!