Thanks for your great support and sorry for the very late answer.
I have no glue why I haven't replied and I actually hate it when people are acting like this in other Forums
Maybe I lost the overview because I had a lot of activities this time.
Meanwhile the underbody and wheel houses are colored based on your hints.
I also tried for a very long time and very intensively to find the last owners of the car because there was no broadcast sheet available.
Also the hints to find the original colour were unfortunately not leading to a positive result.
The car was disassembled very often during its lifetime and all areas to find the right colour are touched meanwhile.
I succeeded to contact all owners back to 1986.
I had a lot of hope that the owner of 1986 could say something about the original state but exactly this owner was not willing to say anything (for whatever reason).
All other owners were very helpful and friendly and the last owner which owned it the last 15 years in the USA sent a package to Germany just like that with nice things related to the car. Just great!
Even I tried it very carefully and friendly it seems I will never hear again from the 1986 owner, which is really really sad because it was the last hope to get the original state.
I hope you accept my apologies for the late reply and I can still count on your great support in the future.
Michael