Force Mac to display in grayscale
Solution 1:
Yes, this can be achieved with AppleScript and Automator.
The setting to automate is located in:
System Preferences > Accessibility > Display > Use Greyscale
The keyboard combination option + command + F5 also shows Accessibility options.
This Apple discussion, Keyboard shortcut for "Use grayscale", shows how to use AppleScript and Automator to create a menu bar item that toggles the Greyscale on/off.
Solution 2:
As of Sierra you can tell Siri to ‘use grayscale’, the closest thing to a shortcut I’ve been able to discover so far.