Back in the day, my white Oyo also suffered some discoloration in places where your fingers hold the device.
The most recent e-reader with such quality issues is last years Pocketbook Touch HD 3. According to some users, the colored backside is quite fragile and can react to sun cream or other hand lotions. Or be easily damaged by hard shell cases.
(Source of the second attached picture: https://www.e-reader-forum.de/ebook-...ng-loest-sich/)