Purple berry

These delicious, sweet, succulent berries are found throughout the Fire Nation. They aren't poisonous, but they will turn your tongue purple.

During "The Painted Lady", Katara feeds Appa with some of those in order to prolong the groups stay in Jang Hui and help the villagers.Sokka and Toph later find out and question them about it, and it can be seen the both Toph and Momo have purple tongues.