There can be a visual difference you see depending on the shell color. I've always put very dark skins on my silver readers, that difference being one of the reasons.
But you'll also find differences in the background color of the screens even on the same exact models when you put them side by side. I noticed it when I got my 2nd silver 350, and you'll find various threads about it. It's just how e-ink screens are, even the identical screen and size from the same batch.