Hi Antonio,

I have updated my blender build including your latest commit and I still see some problem using images as references, i.e stroke colors gets darker and aliased when the image ref is in the backdrop, however when you use objects like plane or even images as plane in the backdrop there is no such issue.
Using my latest build I have used the same file reported in this thread (and just changed the depth of image ref to back) and added a plane in between image ref and grease pencil object and the result you can see in the screen capture.

Further i checked different layout options like general, 2d animation etc..of all 2D animation layout results are better. Later I checked in other layout options and find that by changing the viewport color to between mid grey to white depending on the color of the ref image the result is better. below are two cases of viewport shading one with white and

the other worked at mid grey

In my opinion may be this is a issue with image reference and its viewport rendering..