Only users with 500 karma can downvote.

Both are mechanisms to dull the potency of new users until they have a chance to learn how HN is expected to work.

This might sound gatekeeping, and it literally is, but consider than HN signup is open and takes 15 seconds with no verification. HN likes the way HN works and these provide simple rate-limits on destructive or oblivious change.

YCombinator founders have their own colours which are only visible to each other.

There are plenty of undocumented features like this one.