Hello bazza,
Thanks for your tip.
I have added an alpha channel to the above-mentioned layer, the one where I wish to change the overall color of its background.
Later, with Gimp, I have applied the tool “Select by color” and I have erased this selected color with the “Eraser tool”.
It kind of works but some part of the nematode are erased as well.
Here is the results:
BTW, I have also tried some Gimp “Blending modes” with the 2 layers in the stack but the result is not completely satisfactory. On top of that, I have also tried another G’MIC filter: “CLUT from after before layers” but id didn’t work either [1].
In conclusion, I have noticed that the transfers color advanced filter of G’MIC never works on my images since, I suppose, they are pretty much identicals.
They are all taken with a Leica camera mounted on a microscope and the subjects of these pictures are little worms (nematodes) in a drop of water over a microscope glass slide.
This same G’MIC filter works fine with my Nikon DSLR images though 
For instance, I take 2 different pictures of an apple and then I am able to transfer the color between these 2 images with the filter.