Part of the Rainbow Event
Yellow butterflies are collected and usable during the Rainbow Event only. This event has occurred prior to 2011, from April 17 to May 2, 2010, so it is possible the event will occur again. The 2010 event also involved the use of yellow butterflies for scroll upgrades.
Status as an Item
Yellow butterflies cannot be sold or traded with other players, nor do they have any other statistic. The in-game description reads: "A cute little yellow butterfly. It might fly away if you don't keep your eye on it."
Used for Scroll Exchange
The yellow butterflies were used to exchange 10 percent and 60 percent scrolls for 15 percent and 65 percent scrolls. The exchange could be done through the nonplayer Cassandra. There is no other use for them, nor can they be used outside of the event.
Use of Scrolls
A scroll can be used to enhance either a character's or particular item's statistic (for example, defense, attack, strength or intelligence). The success of this enhancement depends upon the percentage of the scroll. For example, using a 10 percent scroll would have a 10 percent success rate, and a 65 percent scroll would have a 65 percent success rate.