We are a Volkswagen approved accident repairer, along with Mercedes, BMW and many others.
There is a known issue of Plastics and Metal paint. Even if painted at the same time, out of the same mix and air pressure, the dried/baked results can differ. This is because of a number of reasons. When painting metal and plastic, the drying rate is different and the flakes or pearlescent lie down differently causing this effect. Again most basecoats (manufacturer specification) are waterbased so again the drying rate affects the final result. Angles of panels can convince you the colour is wrong where as it's how the light is reflected. We have had hundreds of conversations with our customers so we do a few things.
1: Always take images of the unrepaired bumpers or plastics, so when we are questioned we can show the colour difference before we do the repair.
2: Walk around the car with the customer before we start any work and point out the difference shades of plastic and metal.
3: We have documentation from the paint suppliers explaining this phenominom.
I'm not saying your repair is in this category and it does look light, but this is a well known problem throughout the industry.
Look around some carparks at different vehicles, not necessarily VW, reds, blues, silvers, whites, and you'll see the problem everywhere, especially on new vehicles from the factory.