I'm a recent owner of Kobo Aura One and a newbie to Kobo. Love the size, font customization and the natural light option. But when I received the device, I noticed that there was the same two tone issue in A1 just as the voyage. I was disappointed and contacted kobo. I showed them some pictures and they agreed to send a replacement.
The replacement arrived after several days of follow-ups (and just being a nuisance) and it looked very yellow and had the two tone problem (only much worse) but no green light problem. Contacted Kobo again and they sent replacement 2. Two tone issue again with a small green light at the bottom. Returned both the replacements and stuck with the original as the gradient was comparatively better than the other two. I might try after a few months as I am still very annoyed with the gradient and the green light. Contacting Kobo through their FB was better than using their chat or phone option. To Kobo's credit, they honored their warranty and sent two devices from Canada to US and provided a free return shipping label to Canada. They also provided a credit on my account because of this issue. So although they are slow, if you contact them through FB, they do respond well. The problem seems to be with their QA. For anyone thinking of getting one, try to get it in a physical store rather than online. Or wait a few months and see if the quality has improved then.
I am attaching a link for anyone who wishes to see a comparison.
https://imgur.com/a/CDAG8
http://archive.is/nayGI