The RGB versions of the three black and white images can be used to check the grey reproduction and the black composition of CMYK printing profiles. A sample application is a black and white ad printed in four colours for better contrast.
The CMYK versions of the three black and white images can be used to assess the grey reproduction, the mid-tone spread and the ink trapping of a print run.
The grey scale versions of the three black and white images, in turn, can be used to assess the black simulation of a digital proof printing system. Besides black ink, such systems also use the chromatic colours to simulate single-colour printing with black ink. This can lead to different colour casts in different tonal value ranges of the proof print.
https://github.com/erkin/ponysay