What? You want to understand mermaid casting? You need a quick thread on mermaid skin colour, you say? Well, okay then.

The word you're looking for is 'chromatophore'. Fishes (among other sea dwellers) use these cells to determine skin colour. There are many kinds e.g xanthophores (for yellow) or, hey, melanophores (for brown and black). How random!

Organisms with chromatophores can change colour within minutes. For homework look up 'phenotypic plasticity'. These are used for communication, camouflage etc.

Your deep-sea mermaids (like Ariel in'The Little Mermaid' or Madison in 'Splash' ) MUST have chromatophores. It's necessary for survival.
Fresh water mermaids might or might not, jury is out on that.

So, please and thank you, stop talking about "deep sea organisms aren't exposed to light and won't develop melanin/melanocytes".
Your pseudoscience is showing.

(Plus, there's no such thing as mermaids, but I think we're waaaaay beyond that...)