Shadow Mario Paint
When activated, Bowser Jr. will transform into Shadow Mario, and then paint an orange "X" on the screen. Anyone caught in the "X"'s hitbox will take damage. Bowser Jr. will then be teleported above the top camera boundary and will be able to move around freely while the "X" is onscreen. The "X" deals knockback each time a character walks into it, and the knockback usually sends them towards the middle of the "X", making it difficult to escape. When the attack ends the "X" explodes, dealing relatively high knockback. All hits taken from the paint have the same knockback regardless of damage until just before the "X" clears away.
Shadow Mario was a form Bowser Jr. took in his debut game, Super Mario Sunshine. He used it to graffiti Isle Delfino to frame Mario. Shadow Mario later appeared in Mario Golf: Toadstool Tour as a playable character.
The Magic Paintbrush was invented by Professor E. Gadd (who also invented various other machines, including the Flash Liquidizer Ultra Dousing Device, the Poltergust 3000,the Poltergust 5000 and the Poltergust G-00) and was either given to or stolen by Bowser Jr. prior to Super Mario Sunshine. In said game, the paint used by Shadow Mario was dependent on the level in which he appeared. The glowing orange used by Shadow Mario Paint bears a close resemblance to the paint used in Pianta Village. Paint in this level behaved similarly to lava, dealing damage and causing Mario to jump after taking a hit, similarly to how it works in the Final Smash. It, along with the Junior Clown Car, has become a recurring theme for Bowser Jr., especially throughout many spin-off titles. The Koopalings use the Magic Paintbrush as they are Bowser Jr.'s alts, but they have never used it outside of Smash.
The name refers to Mario Paint, one of the few games to utilize the SNES mouse.