From the Super Mario Wiki, the Mario encyclopedia
Paint Hunter! is a minigame that appears in Paper Mario: Color Splash. It is the secret fourth Spinning-Door minigame, accessible after matching the rainbow "?" panels that appear after using the door once.
In this minigame, Mario is transported into an "alternate dimension" resembling an area he has already visited, with several elements, such as trees and flowers, hidden from view. The possible areas are Ruddy Road, Bloo Bay Beach, Sunglow Ridge, Mondo Woods, Plum Park, and Tangerino Grill. The minigame's objective is to use paint from Mario's Paint Hammer to reveal as many objects in the area as possible within 45 seconds. When the time runs out, Mario will receive 10 coins for every object revealed.
"You've encountered an alternate dimension of...invisible things! Reveal as many things as you can within the time limit!"
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