I have attempted to create a near-human alien that is human but with an unnatural skin tone. In this case purple. I described the hair as black and skin as lavender, just kept making purple haired humans.
It might be worth trying let FF make the hair purple, and as long as everything else is ok, take it to gen fill to change the hair colour to black. Photoshop is another option.
Oh apologies, I didn't explain it right. It actually got pretty much nothing right. The skin tone was not purple as instructed, some were pale, others dark, but not the requested one. I more just was trying to say that it made a skin tone instruction in the hair and ignored the hair instruction. Essentially there's nothing to salvage