I have a Shine 3 and a Vision 5. I use them interchangeably but primarily the Shine 3 for novels and the Vision 5 for work. Both have KOReader side-loaded and it works absolutely brilliantly. There are some very good walk-throughs on
https://allesebook.de/ showing how to get ADB to work.
Both devices are well built and have a good rendering quality. Outside the reader apps, performance can be a bit sluggish but both the Tolino reader and KOReader work really well. I also have Wattpad installed but that is a bit too cumbersome to use here. The frontlight is well balanced on both [my] devices with no bleed. I don't use the colour change, so I can't comment on that. I like my pages white.
Content is managed with Calibre and bookmarks are synced between the two in KOReader with the Progress-plugin.
All in all, this has been a very positive experience. Truth be told, my previous e-reader was an eight year old Nook Simple Touch that finally gave up on me this winter. I concede that this could account for my enthusiasm.
The only drawback for me is a lack of memory in the app partition, limiting which apps I can load.