You say they live in deep water? Well, A) there's only so much I can do if I don't know the depth since it depends on depth, and B) it's likely mermaids will look a lot different than they are commonly depicted.
Seals are mammals, but they don't live deep underwater. In fact, bottlenose dolphins (another species of marine mammal) "typically don't go deeper than 150 feet." (teacher.scholastic.com) As far as I know, marine mammals do go deep underwater (like the sperm whale) but they do not live there.
That being said, even with my little understanding of biology, I can make some guesses. As Lio said, your mermaid's upper bodies will be pale. The skin there will likely be smooth and rubbery, like a dolphin's, and be just as thick-15 to 20 times thicker than a regular human's, with a third layer of modified skin serving as blubber.
So while these mermaids will be streamlined, as they need to be hydrodynamic, don't expect them to be skinny-bitty! But what about the color of their scales? It turns out, deep-sea life follows a strict color pattern:
- Blue near the surface
- Slightly deeper: blue on the back, white on the underside (like a whale).
- At great depths, most creatures are transparent, with red stomachs
- Below those great depths, animals are entirely red or black
- Those at the very bottom are a pale red or cream color
Considering mermaids would have evolved to live in the sea, they would likely follow this pattern. That being said, there are plenty of colorful fish in Levels 1 and 2 that don't, so who knows? They might look like mandarin fish!