We have the ultra white seats in our 2017 S 100D and 2018 X 100D - and haven't had any problems.
We normally don't have anything covering the seats.
But, when we want to protect the seats (when sitting with dirty/sweaty/wet clothes or when putting in a car seat), we use large ultra white bath sheets (the largest towel size) and put those over the seats. To make that easier, we added some snaps at one end of the sheets, that loops over the headrest to keep the towels in place. And when we're done, we'll take them off - and wash the towels, if needed.
At least so far (we've had the S since March 2017), this has worked well to keep our seats in new condition - and the cost of purchasing the towels and adding the snaps is pretty low... And keep them in the cars at all times (usually in the lower rear storage compartment).
We normally don't have anything covering the seats.
But, when we want to protect the seats (when sitting with dirty/sweaty/wet clothes or when putting in a car seat), we use large ultra white bath sheets (the largest towel size) and put those over the seats. To make that easier, we added some snaps at one end of the sheets, that loops over the headrest to keep the towels in place. And when we're done, we'll take them off - and wash the towels, if needed.
At least so far (we've had the S since March 2017), this has worked well to keep our seats in new condition - and the cost of purchasing the towels and adding the snaps is pretty low... And keep them in the cars at all times (usually in the lower rear storage compartment).