I've noticed something interesting. If a dwarf is hauling an item that has no material value and shows up as a black-on-black square when not hauled, then when the dwarf flashes between his tile and the item's tile, the item will take on the foreground color of a seemingly random nearby item. For example:
Those red division signs are barrels made of tallow soap (just tallow soap, no animal type) which when stored appear as black squares. They are all in the "hauled" state in those dwarves' inventories. Two of those dwarves are mechanics, and then there are those seeds you see on the ground there. On top of that, it's possible some of those dwarves are carrying red items in there inventories. Hence the foreground color of the barrels becomes red. In addition, when viewing the dwarves with 'v', they will flash green, likely because of the green plus sign that a creature's tile alternates with when selected with 'v'.
This color change can occur at any moment depending on (based on what I've seen so far)
anything in the immediate surroundings, but only when they are being hauled. It seems to be independent of item type, as I have observed this with doors and other furniture items as well.
However, since items lacking a matgloss don't have any value, this only has value to the modding world in flavor, maybe reactions (that's how no-matgloss items are made, after all). Still, it could be interesting to see what people might come up with to do with this. I don't think it needs to be tallow soap, by the way. I've seen no-matgloss shells show up as blank squares, but haven't observed very closely how different they are when hauled.