Tale of 2 Brothers

<spoiler> The clue in the sky reads:

"BLACK=A RED=N"

This means that the code the brothers used was that black cards represented the letters A-M, and red cards represented letters N-Z. If the trick explained in the story is actually done, it reveals 5 cards. Those 5 cards, when subsituted with their letter equivalents, yield the answer: AMAZE. </spoiler>

To next puzzle: Gravity Lock.