*EPIC DISCOVERY*
It turns out I've just discovered the rarest USDM 240sx color. The color is called Purple Gray Pearl (#LK0). It was only offered on 1991 models, came in both hatch and coupe.
Only 47 total units exist!!! (31 fastbacks, 16 coupes)
I can't even find a 240sx with this color so I had to dig up the 180sx picture you see above. This color was probably overlooked since it can easily look like Charcoal Metallic (and the fact that's its so fucking rare.) In Japan it was offered on the 180sx '91 to '93 year models.
edit:
Only image I could find of the color on a 240sx.