SlackBuildsOrg/system/zsh-syntax-highlighting

zsh-syntax-highlighting (Fish shell like syntax highlighting for Zsh)

Provides syntax highlighing for the shell zsh. It enables highlighing
of commands whilst they are typed at a zsh prompt into an interactive
terminal. This helps in reviewing commands before running them,
particularly in catching syntax errors.

To enable syntax highlighting in the current interactive shell:

source /usr/share/zsh/plugins/zsh-syntax-highlighting/*.zsh

You can also add this command to your ~/.zshrc to always enable it.
NOTE: the 'source' command must be **at the end** of '~/.zshrc'.