Difference between : and # for Bash comments

Question

Today I saw a bash script use a colon to denote a comment. What's the difference between using a colon and a hash mark?

: This is a comment.
# This is also a comment.

For one, I know you can't use a colon for a trailing comment:

cd somedir : This won't work as a comment.

But the fact that the above example works has me puzzled about how : is evaluated.

Answer 1

: is simply an alias for true, and true ignores its arguments:

# Does nothing:
true foo bar etc hello

# Does the same:
: foo bar etc hello

It's not a comment and should never be used as a comment, because all its arguments are still parsed and evaluated:

: This "comment" actually executes this command: $(touch foo)
ls -l foo

or like here, where StackOverflow's syntax highlighting picks up that the command in the middle is actually just text, even if a human doesn't:

: The command below won't run:
echo "Hello World"
: because the surrounding "comments" each contain a ' characters