I've had the new Mastercraft white sidewall tyres on the car for more than 6 months now. I purchased the tyres in 2 batches from different suppliers — the fronts first and then the rears. The production date on the tyres showed that the 'new' fronts were actually a year older than the rears (2020 production versus 2021).
When I washed off the blue protective coating I noticed that the white band on the fronts was already showing some discolouration (a slight yellowing on the outer edge of the white band) while the rears were perfectly white.
I wondered at the time if this was due to the age difference, and sure enough, after 6 months of use the rears have also developed the same discolouration. Do all white walls do this, or is is just the Mastercraft tyres? I'm inclined to think this is due to inferior materials or manufacturing, it seems I'm going to have to learn to live with it.