All of this information is from The Siren's Cove and it's owner Iridessa. I do not own it.
Open Ocean Mermaids: They have large flukes to allow them to travel from one place to the next. Their tails are often blue, so that they may camouflage easier in the sea. They would need a strong tail with a large fluke to be able to travel to different places in the ocean. The water temperature for them would have to be not too cold, and not too warm, so they would have to migrate from place to place during different seasons. These types of mermaids could also be sirens, possessing beautiful and hypnotizing singing voices.
Deep Sea Mermaids or Atlantic Mermaids reside deep below the ocean's surface. Their ribs and bones are much stronger than a humans, being able to widthstand great depths, without their bones crushing. In appearance, these mermaids are not the beautiful ones in the Little Mermaid. They have fangs, and claws, to help catch and eat their prey : fish. Deep Sea Mermaids do not have access to your local Burger King, but what they do have access to is a lifetime supply of fish. The Deep Sea Mermaid is pale in color, with little to no sunlight getting that deep into the ocean. Their eyes are specialized to see in the dark. For a tail, they need a strong fluke, to be able to swim great distances.
Deep Sea Mermaids tails can resemble that of a dolphin or a whale. It is possible however that they have scales. The deepest of Deep Sea Mermaids, are often depicted as monsters, with angler fish antennae, to allow sight, fangs, and hair made out of tentacles. They also can be depicted with little to no hair. It all depends on the mythology you are reading. Deep Sea mermaids are also known to be the most cold hearted of the race.
Tropical or Coral Reef Mermaids have much brighter and flashy tails. They aren't built for swimming great distances, so their tails are more decorative. They dwell in shallower waters. Their diet consists of sea plants, fish, and fruits, depending on how much land access they are granted. A topical mermaid's tears are said to be very delicate. Tropical mermaids are also NOT MADE for colder waters, so it is imparitive for any Tropical Mermaid to stay away from colder climates.
The reason tropical mermaids are brightly colored is simple. Camoflauge. The bright colors allow for them to hide easier in reefs.
However, Freshwater tropical mermaids usually get their bright coloring from iridescence, something that makes the tail appear to change color as the angle of view or the angle of light changes, almost like a hollogram. Salt water tropical mers usually get their tail colors from pigmentation, the same property that allows your skin or eyes to be a certain color.
Artic Mermaids are built to widthstand the cold and harsh waters of the artic. Obviously their skin must be insulated so they won't freeze of hypothermia. An Artic Mermaid's main diet is fish. Their skin is also very thick and pale. Ice mermaids usually have larger eyes to see well, paler skin, and lighter hair colors. Their hair is usually light colors, like white or blonde.They will also most likely have countershading, muchlike a whale or a dolphin. What that would do, iswhen they are swimming and you look down on thewater, the darker side (or the back side) will blend inwith the dark ocean waters. If you are under thewater, like an Orca, you will see the lighter side, (orthe underside), so they blend in with the lightersurface of the water. Artic mermaids would also need to be very fast and strong, tofight off large ocean predators that lurk in the Artic oceans, such as Orcas andLionseals.
Some have the bottom half of a seal, which is called a Selchie. In folklore, Selchies have been known to physically take off their tail and have legs. They are usually found in Irish or Scottish folklore. Selchies cannot stay in human form too long, and the only way if they do is if a human steals their seal coat.
Freshwater Mermaids from what I've gathered appear to be a murky color. They are often depicted withtwo tails. (Like the Starbucks mermaid!) or with thelower body of a serpent. Mostly green, they actually look rather "seaweedish." Most lakes and rivers are very murky, and the tails are a camouflage to blend in with their environment. Their diet is mainly seaweed, sea plants, and fish. Freshwater mermaids are said to be very friendly, though, not very often interacting with humans. Some are said to grant wishes, even.
However, there are a type of freshwater mermaid called "Lady's of the Lake" which are very evil water spirits that take mermaid forms. They are the souls of women murdered before their weddings, and are known to drag down men to the bottomless deep and murder them. They will only be free of their demonic form when their murder has been avenged.