That cannot beat having additional 2 slots for several reasons:
1. Your suggestion is the opposite of convenient UX and UI
2. Incapable of changing colors without leaving the game
3. New users won't be able to customize colors without labor
Such inconvenient solutions were the core reason why I decided to ask for help.
If your mod is using 2-tone exterior and 2-tone interior due to styling choices, you lose ability to color brake calipers and roll cage because there are only 4 color slots allowed for the car.
For this reason I think it is reasonable to ask for 2 more color slots to be added.