From the Super Mario Wiki
Michelangelo's Paintbrush is an item in the PC version of Mario is Missing!. It is the paintbrush used by Michelangelo Buonarroti to paint the Sistine Chapel ceiling (and apparently the only one that he used, according to various people). Its sprite also depicts it with a palette, although it is not referenced by anyone. When Bowser sends Koopa Troopas to storm Rome, the paintbrush gets stolen from the Sistine Chapel, but it later gets taken by Luigi from a Koopa Troopa after he enters the city. He then shows it to Rome's people, and after gathering enough information to determine the paintbrush's owner, he delivers it to the Sistine Chapel's information booth.