Changing purpose of stay from travel to work in Schengen

I am in a peculiar situation that German embassy in my country (Turkey) is also rather unclear about: I have two valid passports, one Green (i.e. allows free travel in Schengen area as a tourist) and one Red (does not allow any visa-free entrance to Schengen area). Green one is gonna expire in a year or so, hence the need for the Red.

So I will be working in Germany starting from September 1, and got my visa for that purpose on the Red passport. While applying for the visa, I asked the officer there if I can get into Germany with my Green and work with Red, she said yes as long as I did all work-related procedures with the Red one. Accordingly, I got my plane tickets on a cheaper and more appropriate time, knowing that the visa may have a later date of entry.

Then, I started doubting the validity of officer's statement and sent a mail to the same embassy, asking for clarification. Their response was the opposite, saying that I can't get in Germany with Green and work with Red, as I would be registered as a tourist. Their response was rather unprofessional, I must say, and rudely dismissed my follow-up question.

So here's my question for friends here: Is it possible to change my purpose of visit within Germany, as I already have the necessary permits? If not possible (or would take too much time), can I just have a two-day vacation in Croatia with my Green passport, and then come back to Germany when my visa starts with the Red passport? Would it solve my issues?

Thank you all in advance!