we often have a lot of different designs for that kind of "creature" AU settings! it's just too much fun to think of different kind of designs and color palettes to decide just ONE to make to an Official Design.

sometimes (often) i use my favorite color (pink) when making such designs, and sometimes we use themed colors or the OC's favorite color.. using Fordic as an example, his personal favorite color is yellow, but we do associate red with him as well, so his merman color palette could be pink (my fave), yellow (his fave) or red (color associated with him).. but depending on the setting, it could really be ANY color :3

usually though, Fordic ends up with red+yellow designs, Cuvier with monochrome or purple or fuchsia colors (homestuck roots..), Pallas with primary red or primary blue, and Nuptus with light blue!

thank you so much, hope you have a nice day too!
(sorry I took ages to respond to this)