I allways have trouble getting proper colors in fullscreen mode, but in windowed mode everything is fine. So i thought about a nice solution for this: There are games with a "fullscreen windowed" mode - basically window-mode without borders.
Of course GeneRally doesnt offer such a mode, but after some Google-research i found this article on pcgamingwiki.com
The first tool offered there (Borderless Windowed, downloadlink in Youtube comment) worked perfectly right away! After adding the GeneRally Process it forces the Game into fullscreen-window after a 10sec delay (this is intended by the program)
Cu Crono