Art Cars are fine, especially if it's one based on a really good idea 👍
The only real restriction is that the design can't be a direct copy of a real livery that already exists.
As an example, if someone want to use the idea of putting a map on a car, like the 1980 BMW France M1 Le Mans car, that'd be fine, but if they tried to recreate that actual car, it'd be against the rules..
The idea behind this is to introduce an element of originality & creativity to the design, without making it
too much of a think piece.