I’ve been seeing this pop up three times in the past few months lately, so I wanted to make a general post: please don’t say your character has melanism, guys, or is melanistic as a mutation?
I can see why it might come up as a possible mutation in-verse, as the inverse of albinism. But albinism is a multi-faceted term that encompasses a number of different pigmentation issues, and which also comes with health issues other than simply lower pigment. While it’s still arguably tasteless, given the issues that people with albinism face in certain areas of the world, it is ultimately a medical condition.
Melanism is literally just an increased amount of melanin in the skin or fur. When you say your character has melanism, you are not only specifically drawing attention to darker skin as something that is abnormal for the species - you are setting a standard in your particular setting that it is actually wrong to have darker skin for trolls.
While a lot of members of the fantroll community are white or light-skinned and may not have to deal with this issue, colourism is a serious issue IRL for a lot of us. It isn’t pleasant to come online and realise that we might have to deal with it in regards to our alien characters of a completely different species as well. While canon features trolls discriminating against one another in a variety of ways in canon, skintones are not one of them.
And honestly, if you’re going to make something that is literally just a physical facet into a notable character trait, it’s worth considering why you want to make this a part of your setting in the first place? I’m always delighted to see fantrolls with dark skin! It becomes iffy when dark skin becomes something that has to be justified.