Slightly hacky script to toggle macOS greyscale mode on / off.

ToggleGreyscale.scpt

tell application "System Preferences"
	set current pane to pane id "com.apple.preference.universalaccess"
	delay 1
	tell application "System Events" to tell process "System Preferences" to tell window "Accessibility"
		select row 5 of table 1 of scroll area 1
		tell tab group 1 of group 1
			click radio button "Colour Filters"
			click pop up button 1
			click menu item "Greyscale" of menu 1 of pop up button 1
			click checkbox "Enable Colour Filters"
		end tell
	end tell
end tell

tell application "System Preferences" to quit