This dungeon is not always available to be played in-game. It is only available sometimes. It is currently available.
"Rainbow Garden is an event-only dungeon. Rainbowmons are used to Evolve Monsters and are graded from 1 Stars to 5 Stars."
The Rainbow Garden is a special event-only dungeon consisting of only Rainbowmon. There is only one stage, which consists of a boss Rainbowmon. Defeating it will cause it to divide into two smaller ones, each of which will divided once more when defeated for a possible total of four Rainbowmon. Clearing stage will award a player with a max-level Rainbowmon of either 2-star or 3-star grade. Once dungeon is cleared once, players may not enter the dungeon to clear it again.