What really confuses me about the outrage over casting a black actor to play the little mermaid is that a mermaid is a fictional creature. Why are people so upset that a fictional creature doesn't fit what their particular preference is?
It should be as realistic as possible. My Ariel is fish colored for the same reason fish are. Maybe greenish blue for water camouflage, or grayish for sea floor camouflage, or brightly patterned if she lives near coral reefs or is poisonous. I'd love a cuttlefish mermaid for the flashing colors but I guess that's not technically a fish, not sure if it matters.