How to enable/disable grayscale mode in Accessibility via Terminal/App?

Install scriptisto and save this script as gray_darwin.c, and make it executable. Also, have Xcode CLI tools installed.

#!/usr/bin/env scriptisto

   // Self-contained, has no dependencies except Xcode CLI.

   // scriptisto-begin
   // script_src: main.c
   // build_cmd: clang -g -O2 -std=c11 -Wall -framework ApplicationServices main.c -o ./script
   // scriptisto-end

#include <stdio.h>
#include <ApplicationServices/ApplicationServices.h>

CG_EXTERN bool CGDisplayUsesForceToGray(void);
CG_EXTERN void CGDisplayForceToGray(bool forceToGray);

int main(int argc, char** argv)
{
  bool isGrayscale;
  if (argc > 1) {
    if (argv[1][0] == 'y') {
      isGrayscale = false; // will toggle this to true
    } else if (argv[1][0] == 's') {
      printf("Grayscale is now: %dn", CGDisplayUsesForceToGray());
      return 0;
    }
    else {
      isGrayscale = true;
    }
  } else {
    isGrayscale = CGDisplayUsesForceToGray();
    printf("isGrayscale = %dn", isGrayscale);
  }
  CGDisplayForceToGray(!isGrayscale);
  printf("Grayscale is now: %dn", CGDisplayUsesForceToGray());

  return 0;
}

Now you can use it like so:

function display-gray-is() {
    [[ "$(gray_darwin.c s)" == "Grayscale is now: 1" ]]
}
function display-gray-toggle() {
    gray_darwin.c
}
function display-gray-off() {
    gray_darwin.c n
}
function display-gray-on() {
    gray_darwin.c y
}

This requires developer tools to compile an objective C program. The program is based on this Stackoverflow answer.

Create a file called grayscale.c with the following contents:

// clang -g -O2 -std=c11 -Wall -framework ApplicationServices
// https://stackoverflow.com/questions/14163788/how-does-on-screen-color-inversion-work-in-os-x
#include <stdio.h>
#include <ApplicationServices/ApplicationServices.h>

CG_EXTERN bool CGDisplayUsesForceToGray(void);
CG_EXTERN void CGDisplayForceToGray(bool forceToGray);

int
main(int argc, char** argv)
{
    bool isGrayscale = CGDisplayUsesForceToGray();
    printf("isGrayscale = %dn", isGrayscale);
    CGDisplayForceToGray(!isGrayscale);
    printf("Grayscale is now: %dn", CGDisplayUsesForceToGray());

    return 0;
}

Then run the following command to compile it:

clang -g -O2 -std=c11 -Wall -framework ApplicationServices ./grayscale.c -o toggle-grayscale

Now run ./toggle-grayscale This instantly makes the screen grayscale or not across all applications, but doesn't update the setting in System Preferences.

Tested on MacOS High Sierra 10.13.6, clang version Apple LLVM version 10.0.0 (clang-1000.10.44.4).

The .plist files which are modified by the Accessibility panel are found in ~/Library/Preferences and they are com.apple.CoreGraphics and com.apple.universalaccess.

No changes are made to any files in ~/Library/Preferences/ByHost

Execute the commands:

defaults write com.apple.universalaccess grayscale -bool yes

defaults write com.apple.CoreGraphics DisplayUseForcedGray -bool yes

Then force the color change to greyscale by unloading and reloading the universalaccessd process:

launchctl unload /System/Library/LaunchAgents/com.apple.universalaccessd.plist

launchctl load /System/Library/LaunchAgents/com.apple.universalaccessd.plist

To switch back to color execute the same commands with the defaults parameters set to no.

Unlike the Accessibility panel, which changes the color instantaneously, this method can take a few seconds to make the transition.