I discovered this last night by accident, but when you use the photo booth in AC: New Leaf to take your profile picture, you can actually make the photo black & white or with a sapia filter. By holding down either the right or left shoulder buttons when the picture snaps, it applies the effect! I don't remember if right was for black & white or left, but try each button and you'll figure it out. A nice little bonus feature!
Adding to this, if you have emotes from Shrunk, you can use them in the picture too lol.