I have ordered tiers, but I did have that problem when buying china. The pattern came in bright white and an off white (sort of creamy, like fondant), but the sellers called everything "white". I had to start specifically asking if it was bright white.
Many/most of the old Wilton plastic plates will have yellowed. There have been many suggested ways to whiten them but I have not found one of those tricks to work for me. One that does come to mind and the person swore it worked perfectly was to put them into a net bag and soak them in her swimming pool!
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