The 2018 Audi RS5 in Forza has a surprisingly accurate colour palette, all 8 colours available for this model year are represented in the game. But if you were willing to pay extra, Audi offered more around that time - Audi Exclusive Colours. A selection of 80 different paints for all Audi models, found here
While Forza is unlikely to add a tab for all colours of a Manufacturer, I wish to see at least some of these Audi Exclusive Colours on at least some Audi models, like this paid DLC RS5.
List of colours that definitely should be added:
- Nogaro Blue Pearl
- Toucan Yellow
- Samoa Orange Metallic
- Goodwood Green Pearl
- Merlin Pearl
- Dark Red Mica
- Bronzo Montecarlo Metallic
- Dark Burgundy Pearl