This issue has persisted since at least Forza Motorsport 6. On the Audi RS4 B5, all Factory Colours bleed through all custom colours when on side mirrors only.
By default, they are supposed to be painted brushed alluminium, which is reflected on all Factory Colours, here is a few
This is how they look like when Two-tone Polished paint is applied
This is how they look like when Matte paint is applied
And of course, this happens with Carbon Fibre as well
The same behaviour can be found on Forza Motorsport 6
I suppose this has something to do with how the factory side mirror finish is displayed and how it interacts with all colours. If a player wanted to apply the factory side mirror finish on a different colour, it’s just not going to work, as shown above.