I like the way you applied color to your line art. Never been daring enough to do that to my line arts. I'd use 2 colored gel pens but that's it. Good job on rendering the anatomy, it's in proportion.
How you rendered the colors and form of the tail fins are cool too. I do recommend the mermaid facing a little bit towards the camera (side view) just to add a little more appeal as one part of her eye can be revealed and those are parts where people would usually look at when viewing characters from a drawing.