How to make `ls` color its output by default, without setting up an alias?

There is no way: the ls man page will show you that the default setting (for --color) is 'none' - ie. never use colour.

Any reason you don't want to use aliases? I'm a recovering Red Hat user, so every time I install a new distribution I set three ls aliases like so:

## Colorize the ls output ##
alias ls='ls --color=auto'

## Use a long listing format ##
alias ll='ls -la'

## Show hidden files ##
alias l.='ls -d .* --color=auto'