From the Super Mario Wiki, the Mario encyclopedia
The Dimensional Doors are seven doors that appear on the Flipside Tower and one door that appears on the Flopside Tower in Super Paper Mario. The doors appear when their corresponding Pure Hearts are placed inside a Heart Pillar. Each new door leads to a dimension where the next Pure Heart is located, save for the last one, which is meant to lead the Heroes of Light to the location of the Chaos Heart and the one who wields it.
The colors of the doors are listed below. They each correspond to the Pure Heart that created them, except for the final one, which corresponds to the Chaos Heart.
The first seven doors are found on the Flipside Tower. The door that leads to Castle Bleck, bigger than the other doors, is on the Flopside Tower.