Within Puzzle Swap, there are pink pieces which cannot be obtained by Playcoins. Nintendo has said that they can only be obtained through other players by Streetpass. But here's the question: If other players can only get their pink pieces from other players, when are pink pieces ever generated? How does the first pink piece become obtained by anyone?
The first time a panel with pink pieces is unlocked, you will receive a free random piece, which can be a pink piece or a regular piece.
Apart from that, meeting other players with those pieces, or being a special employee at Nintendo, there is no other way to unlock them that comes to mind.
When you get a streetpass from a Nintendo worker, they should have all the pieces, so you should be able to get a pink pieces.