ToggleGrayscale.scpt

-- Line up a Python script for dynamically loading the private framework 
-- and invoking the required private methods to get current grayscale mode
-- and set it to the opposite value.
set toggleGrayScript to "python -c 'from ctypes import cdll
lib = cdll.LoadLibrary(\"/System/Library/PrivateFrameworks/UniversalAccess.framework/UniversalAccess\")
lib.UAGrayscaleSetEnabled(lib.UAGrayscaleIsEnabled() == 0)
'"
do shell script toggleGrayScript