Some might find this interesting, a comparison between my two Sage.
First Kobo Sage from Fnac (on the left), second from Amazon (on the right) :
https://drive.google.com/drive/folde...O4bWC4CJQe3u8P
The one from Amazon is much better IRL, and IMHO (both with or without comfort light).
Without comfort light : Amazon = Paper White VS Kobo = Creamy/Greenish White
With comfort light : Amazon = Warm Orange (red is more pronounced, which I consider is a good thing) VS Fnac = Greyish/Greenish Yellow.
In both modes, the gradient appears to be quite invisible with the one from Amazon (because more subtle, and not a big difference between two opposite sides).
With the one from Fnac, you have to clear your mind to unnotice the major difference between the cold white coming from the button's side, and the creamy white from the other side.
So another big win for Amazon's Sage, which is clearly more uniform (probably less obvious with pictures, than it really is).
0/10 for Kobo; still a bad quality control, and getting a good screen is like playing the lottery.
EDIT : I've added one more picture, comparing both with my Kindle Voyage.
It is interesting to notice that the Voyage is reddish, the Sage from Fnac greenish, and the Sage from Amazon more balanced overall.
Another worthwhile point : contrast is almost as good as Voyage's (unbeatable) one.