I guess the nightly build are signed with an different certificate, maybe we could change it to have a different app id in the future so you can install both apps side by side.
@Hocuri and @link2xt should know more about the nightly builds.
Good idea, feel free to open a feature proposal for that here on the forum.
Yes the color hash function has been updated by @link2xt to output more unique colors: Changeable Contact Icon Color - #6 by link2xt
Currently it saves only core data, the User Interfaces also store some config elsewhere. There were plans to allow the UIs to store their configuration in the core-accountdata (account database config table) too, but nothing super concrete yet.