The Rainbow Colonies are new undiscovered territory found by the Ranbio Brothers. It is said to house the Rainbow Shards, which are powerful artifacts being pursued by Dowser and his unknown accomplice.
The Rainbow Colonies were first discovered by Ranbio and Colorigi. They broke the news to Quario, who told the Mario Bros. and co. about it. After sending them to the Rainbow Colonies, Ranbio and Colorigi ditched the group. Quario then told the Mario Bros and the rest of the group that Dowser and an unknown enemy will try to obtain the Rainbow Shards in order to take over the Fourth Dimension.
According to Quario, there are 4D artifacts called the Rainbow Shards. Dowser however, plans to obtain them in order to take the 4D for himself. [More info will be added]