The vivid smoolk specializes in vivianite. Though vivianite can be found elsewhere, the equator is an impassable barrier for range expansion.
Because vivianite is simultaneously a metamorphic, sedimentary, and igneous mineral, it (and vivid smoolks) can be found in many places. However, vivid smoolks are fairly sensitive to heat and light, and thus cannot cross the equatorial barrier. The greatest concentration of vivid smoolks can be found in the area that was formerly the Penumbra Bog. The second greatest concentration of vivid smoolks can be found in the area that was formerly the Penumbra Polar River.
Vivid smoolks form "shells" by extruding minerals from their diet. In other words, their shells are made of their own "excrement." The shells are not quite composed of vivianite, since the vivid smoolk metabolizes much of the iron in the original vivianite. However, the shells' color is similar to vivianite, and like vivanite, the color changes depending on exposure to light. As vivianite becomes darker depending on exposure to light, and vivid smoolks are always exposed to light, one can roughly tell the age of a vivid smoolk by how dark its shell is. However, while the color of vivianite changes within minutes, it takes days for vivid smoolk shells.
Unfortunately for vivid smoolks, the color change can be their undoing. As their shells became darker in color, it absorbs more light, making the vivid smoolk hotter. If its shell is too dark, or the environment unusually hot, the vivid smoolk can overheat and die. Due to their iron content, the smell of a vivid smoolk "roasted" in its shell is similar to that of rust, pennies, or (human) blood.
Incidentally, the chemistry behind their shells' color makes the vivid smoolk's photosynthesis more efficient. It receives slightly more energy from the process and produces more water and oxygen. The water and oxygen, as well as the spores, are released from tiny pores in the vivid smoolk's shell.
Vivid Smoolks' color usually ranges from pale green, to blue-green, to dark blue-green, to indigo, and then black. On rare occasions (depending on ingested impurities) the shell may be purple. Such shells are roughly as rare as albino animals.
(No, I did not make a mistake in their diet. Their shell prevents them from eating detritus that settles on their surface.)