Not allowing third party apps to adjust screen colors seems like a reasonable security boundary to me. For the most part when you close an app on iOS, it gets closed. It doesnt get to keep changing system settings in the background. Would be awful if in addition to notifications, apps also got to adjust your colors.
Screen tinting like that is exactly the kind of thing that should be an OS-feature, not an app feature.
They are similarly quite restrictive on MacOS, with some system-impacting features being locked behind “accessibility” permissions. So that arbitrary apps can’t interact with other apps unless they are actually doing something that needs it like “being a screen reader”.
iOS doesn’t have the same sort of permissions. Apps can’t take over interactions with other apps, or change display settings, etc. This is a security boundary. And changing that specifically for “changing screen colors” seems unnecessary to me.
For context, as a software developer and Mac OS user who also happens to daily drive a screen reader, I seriously doubt whether you could implement a third-party SR on that platform.
It seems that third-party software, even software with accessibility permissions, doesn't work on password screens (and probably in a few other similarly-secure places), and you need those to be accessible. Not to mention weird places like system recovery, which (for very obvious and understandable reasons) does not allow 3rd-party software at all.
I guess you could use a third-party SR for most of your system and then toggle VoiceOver on when accessing the secure parts, but that would get very annoying very quickly.
There's also no 3rd-party access to some speech-related features, like the higher-quality neural Siri voices. You'd also need APIs for things like automatically being informed of incoming system notifications to read them as they come in (which the first-party VoiceOver does), and those don't seem to be available at all.
Screen tinting like that is exactly the kind of thing that should be an OS-feature, not an app feature.
They are similarly quite restrictive on MacOS, with some system-impacting features being locked behind “accessibility” permissions. So that arbitrary apps can’t interact with other apps unless they are actually doing something that needs it like “being a screen reader”.
iOS doesn’t have the same sort of permissions. Apps can’t take over interactions with other apps, or change display settings, etc. This is a security boundary. And changing that specifically for “changing screen colors” seems unnecessary to me.