I just received my Colorsoft and unfortunately it also has that yellow bar at the bottom. In fact it's more than a yellow bar, the whole screen has a pretty visible gradient which ramps up rapidly at the bottom of the screen.
For those interested I attach a couple of pictures of my Colorsoft's screen at full brightness on a white background, one with the left half flipped vertically, and the other one with the bottom half flipped horizontally, so the gradient can be observed more easily. The gradient is not uniform, with lighter blotches around the center of the screen. Overall it seems to be slightly better horizontally than
my Libra Colour, but significantly worse vertically.
The Kindle does seem to handle dark mode (or "black page color") significantly better than the Kobo, with a much more uniform look on a dark background. As for color rendering, it's a bit of a toss-up; out-of-the-box I prefer the Kindle in vivid mode, but Kobo offers more rendering modes via the developer menu.