Hello bazza,
First off, THANKS a lot indeed for your help! 
Today, I took another 2 pictures with the Leica camera mounted on the microscope. They are about the same fungus (Fusicoccum spp.).
The first is darker; the second is much brighter.
In short, with the G’MIC transfers color advanced filter I have tried to make the dark image much brighter.
With Gimp it is extremely easy and it suffices to work with the curves tool (it only takes a few seconds). On the contrary, it is extremely difficult to get the same result with this G’MIC filter.
Here is the darker image, that is the one I want to brighten as much as the second image:
Here is the brighter image (the “reference”):
