When Forza gifts people those cars with special liveries (Xbox Porsche 918, New Year Camaro etc.)
Usually people just sell them or just remove the livery.
The cars should be either called for example:
Porsche 918 Xbox edition
Chevrolet Camaro Supersport New Years Edition
BMW I8 Earth Day Edition
Or be unsellable and that the special livery is the default color, like the Oreo Zonda
Is this so that you dont accidentally remove the vehicle or to make them collectible. Because if you do remove the vehicle i do believe those liveries can be found in the liveries section for download from its creator (as long as they still have it shared). However if this is more to make them collectibles i personally find this game already has too many cars that arent much more than just collectibles but thats just my opinion