Red Crayon

For the monster from which you'll get +10% exp. I just finished getting the crayon and placing it on the rainbow thing.

Does the monster you get +exp get set as the first monster you kill?

EDIT: http://www.southperry.net/showthread.php?t=26317

Cassandra offers you options.

Seems like you found the answer. You can change the monster too if you want, btw.

Just note that monsters you get differ depending on where you're talking to Cassandra. If you're on Victoria Island, you'll only get the monsters there. You'll get monsters from the other regions if you talk to Cassandra anywhere else.

I haven't tried this in the World Tour areas or found a way to have Neo City/Kerning Square monsters appear yet, although I would imagine that this event doesn't take the monsters in those areas into account (this being an event that originated in KMS, and I'm pretty sure this was made before the release of Kerning Square/Neo City).