fish, the friendly interactive shell, is a commandline shell intended to be interactive and user-friendly.
fish is intentionally not fully POSIX compliant, it aims at addressing POSIX inconsistencies (as perceived by the creators) with a simplified or a different syntax. This means that even simple POSIX compliant scripts may require some significant adaptation or even full rewriting to run with fish.
zsh with oh-my-zsh addon can do the same amount of pretty colours and qol stuff, with the addition of being POSIX compliant. Not that fish is bad or anything, but you don’t want additional troubles with random incompatibility on top of the usual learning curve.