potion brewing is a subsect of witchcraft with roots as far back as ancient times. These potions are made to serve various purposes in different fields. Curing illness, making an individual fall in love, increasing one's strength, and other effects are marketed to the public or to other witches. They are brewed in large pots called cauldrons, in order to produce a large batch all at once. Various items are included, boiled, and stirred in the pot to make the concoction (eye of newt, a dead infant, granite skin of a 500 year old dead turtle that was born on a solar eclipse, etc). The potency of the brew will depend on the skill of the witch and the quality of the items. While this is a necessary profession in witch society, brewers are often stigmatized by their fellows.
Potion brewing has negative effects on the creator's skin. One condition is that it turns their skin green. The condition takes place slowly over the course of years. The more time a witch spends making potions, the more complete their skin change becomes. Eventually, They become green from head to foot. The skin also changes its texture, becoming more sunken and hanging off the individual's bones. As anyone would tell you, its not easy being green. These unfortunate souls are somewhat held at a distance and seen as greedy, as they are "green with envy". This stigmatization of society often leads many witches with the condition to go insane, taking to fits of cackling laughter.
Changes in melanin production can be due to a variety of conditions and some medications. Skin darkening can be due to changing hormone levels or medications, but it can also occur from exposure to ionizing radiation (such as the sun) or heavy metals. Radiation therapy can also cause an increase in skin pigmentation. However, all of these can have potentially kill the victim. Also, skin color can only go from darker to lighter, with no other colors being possible. It's not like we see orange or blue people running around these days. How can brewing potions end up having these affects on the maker's skin without having fatal results.