These prints are the result of experimentation with colourisation
Photogravure is intrinsically a monochrome photographic process. However artists have always experimnted with colour and the method by no means confines you to using only black ink.
Methods of colourisation include 'A la Poupee' - using different coloured inks on different parts of the plate, 'Chine Colle' using the pressure of the roller to embed coloured tissue into the paper. It is also possible to do multipass pass prints of CMYK channels of a colour image etched on to four separate plates