If the Aura can't legally be attached to anything, you won't be able to put it onto the battlefield, and it will be put into your hand by the next part of the effect
Each Core Set from 6th Edition on will have a set symbol, just like all the other sets after Arabian Nights
To balance this color requirement against the draft process, when a player makes a pick from a pack, they take two cards
On the other hand, placing a card underneath a Whisper when you cant see the cards means you risk losing what might be a pivotal draw
Imagine rounding a corner
In any fashion, singleton is my method of choice, because I think it mimics the way most people enter the game: a relatively small pool of cards, rarely any meaningful duplicates, and a hunger to just play the game