That reminds me somewhat of the "blink comparator," a tool used by astronomers (before the days of computers and CCD cameras) to look for differences in images. One was used to find Pluto, for instance.
I am not at all saying that a graphical comparator tool is useless. Far from it!
What I am saying is: wouldn't it be great if you could select a trace and have the graphical-compare tool tell you the net name and display its changes? This would make the tool much more useful. "Hey, how did you re-route that LVDS pair xyz_p and xyz_n?" Click on the trace in either the new or the old image, then the same net is highlighted in the other.
To do this, you can start with the kicad_pcb file, which has all of that netlist information, plus it's in a relatively-straightforward text format which can be parsed by any number of tools.