Well, that's not the best option. So you can simply recalculate the cost of a car in VPE, sell it and buy a new one with a new interior color. And then return the normal value to the car.
It would be nice to change the interior color in the game. How the wheel rims were changed in the workshop. But, here again a question to the game engine.
By the way, the question of the limited number of slots in the game's database, about a year ago, I decided a banal instant substitution of the game's database. I needed this so that it would be possible to add cars to the game indefinitely, but I think it is clear that this approach allows solving other problems of the limitations of the game database.
Now I don't think about restrictions at all!
This is how it works:
This is just advice. I hope no one will be offended.