I can’t find the thread now via search, so this is from memory. I believe the thread was questioning whether one could remove skin tags by buying a cryogenic substance. Although I am not prone to skin tags, I did have one at the time. In the interest of science, I used butane to freeze my skin tag. It took a couple of weeks and 2 or 3 freezing applications for it to fall off.
Emboldened, I decided to treat some subcutaneous cysts (probably not the right term, but basically persistent pimples under the skin that cause reddening of the skin.) Surprisingly, the freezing treatment was effective but has left darkening of the skin. It has been several months and the darkened skin (looks bruised) has lightened a bit but is still discolored.
The question is whether I can expect a normal skin tone to return eventually? (BTW the areas are on my posterior and the middle of my back.) If I have permanently affected my pigments, should I jump in a vat of liquid nitrogen to get an even dark color? 
Note: In the original thread, it was decided that in general it is not a good idea to self treat yourself as I did.
but the most likely explanation is post-inflammatory pigment alteration. We see this often with cryotherapy (using liquid nitrogen on the skin). The result may be skin that is lighter than surrounding skin(hypopigmentation) or darker than surrounding skin(hyperpigmentation). Most often I find hypopigmentation (melanocytes are more sensitive to cold temperatures than other elements in the skin). Perhaps butane gave a less thorough freeze, didn’t kill melanocytes, but initiated inflammation that stimulated excess pigment production.