I'm currently writing a medieval fantasy novel where one of my main characters is a human who can shift into a dragon that breathes a black colored fire. I would only call it black fire for the sake of simplicity, as it isn't a literal black colored fire, and it should be explained as an added pigment that makes it appear dark.
I would also like to know if say, this dragon blew fire upon different substances (like different metals, earth, wood, etc...) would this pigments color be affected drastically? Also, if this pigment is obtained by diet, or some biological chemical reaction, or an external environmental factor. In addition to this, how would this dragon and other dragons be resistant to this pigment (in the sense that they would not be wounded, or inhaling a sort of poison), as well as other animals/plants in the environment around them. I would much prefer to give an actual explanation, instead of giving a vague answer of 'it's just magic'.