Rainbow Jellys can be matched with Jellys of all colors! Use them to make a two different color Jelly combo.
After connecting a long line of red Jellys, connect to a Rainbow Jelly, and connect to any group of Green, Yellow, Pink or Blue Jellys. The Rainbow Jelly allows you to make incredibly long connections.
Buy it by tapping on its icon at the bottom of the screen. Activate it any point in the game by tapping on it and choosing a Jelly on the board.