From the Super Mario Wiki
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 on random occasions.
In this minigame, Mario is transported into an "alternate dimension" resembling an area the player has previously visited, including Ruddy Road, Bloo Bay Beach and Sunglow Ridge, with several elements, such as trees and flowers, hidden from view. 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!"
Names in other languages