An unusual side effect of the color changing torso is that Zartan's torso is molded into one piece instead of two halves. Ordinarily, the two halves of a G.I.Joe figure's torso is held together by a screw in the figure's back. However, the two halves of Zartan's torso are molded together. Removing the screw has no effect on the figure.
if im wrong i appologize.
The Zartan, Zarana, and Zandar torsos are obviously initially molded in two pieces as once you cut the seam open they are just a normal torso inside with all the pegs and the screw mount. Also the head and arms could not have been added if it were molded in one piece. From my observation of separating my childhood Zartan for repair, it looks like the color change plastic is applied to the torso after it is assembled as a coating which fills the seam and glues it shut.
It's been maybe 15 or 20 years since I did it tho. Zandar and Zarana I've bought since then, so I haven't had need to dissect them yet. But process is simple, heat the torso and then trace the seam with a razor sharp tool several times until you slice through the surface plastic and free the two halves. Treat it as a normal figure from there.