That's a very old thread. I didnt even knew about it. O.o It would be a miracle if you can unburrow that tileset.
I'm not entirely sure, but since you use inverted colors for the magmaproof versions, wouldnt it look a bit weird if you have a white or light-grey magmaproof inorganic, and next to it a dark-grey or black non-magmaproof inorganic?
The ingame color might influence the end result and people cant tell if it's a dark magmaproof or a bright non-magmaproof material.
Reactions automatically detect if a material is magmaproof or not, most players know these materials by heart. Not sure what my alternative suggestion would be, (using an icon would look weird), but maybe dont differentiate the inorganic walls based on magma/nonmagmaproof?
Maybe it's enough to just do that for the item. A boulder looks like a boulder, but a magmaproof boulder has a little volcano icon? A fuel boulder has a little flame? That way people can see it directly on the items they have in their stockpiles, but the inorganic walls/caves/caverns still look natural, without any ingame help-icons.
EDIT: Sorry for being a bit incoherent.