Tale of 2 Brothers
From Vanishing Point Wiki
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.
To next puzzle: Gravity Lock.
