Rachel are you talking about a “car magnet” that was left in place for an extended period or a “fridge magnet” that was “tagged” on a car in a parking lot and removed within a short period of time?..
There is no doubt that a “car magnet” left in place for an extended period can cause problems…But actually it is the paint not under the magnet that changes colour (fades) due to the suns rays…Much the same way as a picture frame on a wall…If it is in direct sunlight for an extended period and you remove the picture frame there is a distinct difference in colours…
Now as far a fridge magnets on cars…I have seen media reports of claims made against distributors like me and suppliers, however, I do not know of any instances where car owners have prevailed in court…