By Dan Katz, with art by Elizabeth Bagby

Answer: a red TACK

On three of the walls of the party room there are large posters with
near-identical pictures of a street scene in Normalville. Each of the
three (which are marked as red, green, and blue) are different from
the other two in exactly four places. The fourth wall has a grid in
which each square contains three colored letters. If you pick out the
squares in which one picture differs and choose the letter matching
the color of the differing pictures, you get:


This message spells out ASK US FOR THIS in the shape of an arrow
pointing toward the upper-right corner of the grid, where there is a
red tack. If you ask a Normalville Councilman for a red tack, we give
you one, and this is the answer to the puzzle.

[I can add the differences in each picture later, if we want them.]