So my old phone was an iPhone, and it had the ability to do a more custom color correction where I could make it only use red.
I do this because its more effective for preserving circadian rhythm than "amber tint," and it is easier on the eyes in the middle of the night and better for preserving night vision.
It wasn't perfect - that would be if it used greyscale, but then only used red instead of RGB/white.
This would also be useful for people working in chemical photo development darkrooms... Which is niche, I know.

Does anyone know of a way to accomplish this on android? I find this has turned out to be a surprisingly bothersome quality-of-life feature to lose.

    Dumdum

    I have, and I've also tried combining Night Light with Extra Dim, but the dimness to make the amber-tinted "white" not zing my eyes / screw up my night vision / throw off my circadian rhythm is... Basically off. And thus too dim to see at all.

    (Tangential digression to explain why I'm so persnickety about wanting only red light: I work night shift, so I have to be a lot more particular about this kind of stuff than most people are... For example, I have red led lightbulbs on dimming socket adaptors in some of my lights at home so I can use only juuuust enough red light when I come home after work, to help me go to sleep sooner.)

      Oh, hey, I just thought of another use case: people using android devices while working with telescopes or night photography. Still very niche but it led me to some forums with other suggestions from astronomers... anyway... I found an app on Aurora / Play Store called Night Filter that does exactly what I need. It is, unfortunately, not FOSS. I can probably make it behave itself with mullvad's adblock, not giving it network permissions, and I think I can isolate it from other apps with RethinkDNS? But now we have a solid example of what I'm looking for, in case anyone sees this and can point me in the direction of a FOSS alternative.

      About the only FOSS option I have found is Red Moon, but it is only being barely maintained for basic functionality on the dev's particular phone, which is a Samsung. I don't know enough about security to know if this sort of app would increase attack surface and if said surface would be particularly vulnerable due to it being minimally maintained.

      7 months later

      I'm wondering why this isn't more often requested.
      I had this on an older phone running a custom rom and I loved this feature!
      Seems like someone already requested this on GitHub: https://github.com/GrapheneOS/os-issue-tracker/issues/3718
      @GrapheneOS How likely are such feature requests to be implemented?
      Besides some Python and Java, I don't have much knowledge of OS development...but I would love to contribute one day.