Hi André;
Unfortunately unless you you use the old "Windows Classic" style, the background colourization on Tab Pages does not take effect - even though you can set that in either the Window or User Object painters. I believe that there is already a ticket in the Support System for this issue.
The only thing that I can think of to address this short coming in the near term would to build your old Tab Page user objects like we did way back in PB 3/4 (before the Tab Control was introduced). Then you would have absolute control over the GUI in that case. However, I am not sure if that is realistic logistics wise considering the effort and time.
Regards ... Chris