Is this what you are trying to get?
I did this in a few moves
Using the same picture in Gmic, I made three copies/layers
1. Did Black on White on one
2. white on black on another
Moving white up to the top I changed the blend mode to grain merge so it went 'grey' (neither black nor white) Then merged one and 2 together.
Lastly I overlayed the merged layers on the coloured picture and got this.
Exported as a png to my desktop