Not to alarm you, but in the interest of full disclosure.
It is my understanding that there is an individual bias adjustment on every eInk screen that is adjusted and set as part of manufacturing that more or less determines the "white level." Due to this the greyness of the background can vary from screen to screen and I would think it could even change if the value of the component(s) which determine it change with time.
There is also the infamous "fading in sunlight" issue which I've personally never seen.