What you are looking for is called Syntax Highlight. Are you asking this for the terminal? I think this is a property of the shell. Fish has it builtin and for ZSH, there is a plugin: github.com/zsh-users/zsh-syntax-highlighting I don’t know if its possible with Bash.
If you use zsh, there is zsh syntax highlighting plugin. For bash, a cursory search gave me ble.sh which looks interesting. And as other threads have mentioned, fish shell has this built in, but beware fish shell syntax works drastically differently from other POSIX shells
What do you mean by won’t work? chsh is not changing the shell of the entire OS, it’s changing your users login shell. Unless you’ve done something to make your login break without bash, this is completely fine. Using chsh is even how the Fish docs recommend setting it as your default.
What Ubuntu did isnt about login shells, they replaced /bin/sh with dash, meaning any scripts that try to use sh will instead use dash.
chsh just changes the shell when you log in to a shell. all the other shells are still available and usable. any script starting tieh #!/bin/bash will still run with bash, even if you’re using zsh or fish.