A mermaid's body is tapering, and her tail is like a dolphin's, or perhaps like a seal's. In any case, her tail is not scaled. Mermaid coloring seems to be varied. Their tails are said to be reddish-grey, dark brown, or even speckled. I contend that it is.
Human skin comes in pink, yellow, brown, reddish brown, and black.
In fact, there was even an American family who had a genetic protein deficiency in their blood. Even the two-toned coloring of the traditional mermaid is not without precendent. Blue and green hair are not out of the question either. I have seen cats who were a shade of grey that looked blue or green. Glossy black hair can look blue when the light hits it a certain way.