Standalone tool using private Apple framework to toggle display grayscale mode

gray.c

#include <stdio.h>

// Compile with cc -o gray gray.c -framework UniversalAccess -F /System/Library/PrivateFrameworks

extern void UAGrayscaleSetEnabled(int isEnabled);
extern int UAGrayscaleIsEnabled();

int main() {
        UAGrayscaleSetEnabled(!UAGrayscaleIsEnabled());
}

Want to avoid private framework ? You can do this using private API from public framework (I'm using it since 10.3 at least):

#include <CoreGraphics/CoreGraphics.h>

extern Boolean CGDisplayUsesForceToGray(void);
extern void CGDisplayForceToGray(Boolean gray);

int main() {
  CGDisplayForceToGray(!CGDisplayUsesForceToGray());
}