Tolerance level is very much boils down to personal preference.
It would be too tedious to remove the existing paint. The labour probably will cost the same as a new raw bumper.
There are 2 factors playing against you at the moment
1. Bumper - new paint, while the rest of car - old paint
Colours, regardless car paint, wall paint, printing on brochures, newspaper etc will also fade with time and UV exposure, RED being the most susceptible to fading with UV exposure.
So even if the SAME batch of paint from Germany was used, it will still appear different than the rest of body if sprayed on several years later
2. Substrate - Rest of car body is metal. Bumper is PU (thermoplastic). The visual of colour is also dependent on the substrate (Base) that it is printed on and being of different material, the final effect will appear different. Thats why most makers would first spray a white Base coat on all surfaces prior to the actual colour. It helps to lessen the differences.
Metallic colours (esp with pearl effect) are one of the more difficult colours to re-create. The reason is because the metallic powder (or mica pearl flakes) will vary in sizes from different sources and/or different batches.
Different size/origin/% of powder will reflect light very differently and this is probably whats catching our eyes in noticing differences in colour tones between the new and the old.
Sorry for the lengthy post/explanation but short of spraying the whole car, I'm afraid the other option is to lower your expectation as a perfect match in this situation is not an easy task for any workshop .
truly appreciative of your sharing, especially from someone whose job is also related to colours.
in the last 3 days, I also did a lot of research online and saw similar explanations of paint behaviour.
even national level body repair organisations also emphasised that some colour differences are to be expected.
I also read that white, red and silver are some of the more difficult colours to match.
and you rightly pointed out that metallic colours also pose a huge challenge. Mine is metallic too.
the contacts I spoke too so far advised me to just leave it. They think that the discrepancy is within reasonable level. But my eyes are just too sharp to magnify the difference.
I went back to that workshop again for them to polish off 2 imperfections. They did offer to respray the left side of the bumper to make it appear more blended with the left fender. The right side looks ok though.
but I am concerned that if they do it without stripping the original paint. Such a second-time job may not look as refined and could have durability issue too.
also, there is no guarantee that the respray will be much closer match than the current one.
taking into account the risks, I am now more persuaded that perhaps leaving it alone will be a better option.
by the way, the workshop told me that if it’s a brand new bumper, they will spray it separately before attaching it to the car body. But for existing bumper, they will spray without taking it off the body. Is this an industry practice? In my case, it’s a new bumper.
