The plan was to keep this in testing and not mention it, but I messed up and put it straight in “main” G’MIC gimp branch under Colors 
(as “Transfer colors [patch-based]”).
Well now that it’s there I may as well explain it a little; it’s based on the -patchmatch command and works by first creating a CLUT, which can be output to a layer as well. In fact you can use it with a single image to generate a CLUT only.
The extract CLUT part can also be done from command line using "-gcd_extract_clut "
Ah, I’ve seen indeed activities in your .gmic file these last days. That looks really interesting, I’m ready for testing this 
Thanks Andy !
There’s definite room for improvement, but the idea seems worth investigating… the -index and -patchmatch commands have some things in common. This filter appears best suited to reducing the palette, e.g. from colour to sepia. If you have any ideas about it, in particular the blending (I resorted to a simple -blur on the clut), I’ll obviously be glad to hear them!
