Peugeot 308 2009: Paint defect

My wife's 2009 Peugeot 308 hdi has a paint defect. There is a section measuring 7 inches x five inches on one of the plastic front mudguards that is a darker colour than the rest of the car. We have shown it to two dealerships, who have told us that it is normal for plastic to be slightly off colour compared to metal. One acknowledged the colour difference, but said it was acceptable? Where do I go from here?