What? You want to understand mermaid casting? You need a quick thread on mermaid skin colour, you say? Well, okay then.
The word you& #39;re looking for is & #39;chromatophore& #39;. 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 & #39;phenotypic plasticity& #39;. These are used for communication, camouflage etc.