I figured it out after a few minutes poking around.

You'll need some way to set "secure" settings – if you're not rooted, the easiest way is via a Tasker plugin called AutoTools. Once you've set that up, create a task with an action of "AutoTools Secure Settings", then configure it as follows (under "Custom Setting" in the AutoTools menu):

    - setting type: secure
    - name: accessibility_display_daltonizer_enabled
    - input type: toggle
    - value: 3

That task then toggles greyscale mode, so you can hook it up to an application context or whatever. (Switch the value to 1 if you want a task that always enables greyscale, or 2 to always disable.)

If it doesn't work, diff the output of `adb shell settings list secure` from a PC with greyscale mode enabled and disabled, and use whatever name you get there.

I recently saw an app (Android only) that apparently is capable of setting your phone to grayscale, with the exception of certain apps.

Unfortunately I can't find it anymore. I've been trying hard.

I too would love to live on a black and white phone with certain exceptions (in my case mainly camera and maps).

If anyone knows/remembers this app, you'd make my day :)