As others said, it's most likely just the difference between individual screens. I've had my 350 for about 10 months and love it, so much that I just bought another one as my spare since I don't care for the T1. And there's a very noticeable difference between the 2 screens... the new one I just bought has a lighter background and darker text, even when settings are identical. I noticed it as soon as I booted it up the first time and had to get my old 350 and set them side-by-side to be sure.
There seems to be quite a bit of difference between identical e-ink screens apparently. It's not that my old 350 isn't as readable as the new one, it is. But the contrast seems so much better on the new one, I'm tempted to use that full time and set the old one aside as the spare
.