Usually, fish colours and shapes match their environment.
There won't be much difference from one ocean to the other, important is whether a fish is benthic (flat and often mud coloured), pelagic (camouflaged with dark back and light belly), living in freshwater under plants, in a reef, etc.

There are also signal colours with venomous and other dangerous species.
Personally, I have an idea as to where and how the mer lives depending on tail colours/shape, at least with my own tails.
When I see a luminescent tail with lights on, I automatically go "deepsea mermaid".

But not everyone has a story for their tail or wants one, I mean, people like all sorts of tails but don't want to be restricted where they can use it story-wise.