Instructions
Defeat the Mad Doctor's machine once Mickey escapes from the table he was strapped to. Gus the Gremlin will show you two control panels that must be destroyed; hit each panel three times with a spin attack to disable it, shutting down the killer machine.
Travel down the castle hallway after watching the cutscene explaining how Paint and Thinner are used. Fill in or erase items using the "B" and "Z" buttons on your Wii Remote and Nunchuk. Erase the door frame at the end of the hallway to cause the door to fall over and provide you with a way past a river of Thinner.
Move across the courtyard to the ramps leading to the courtyard bridges, defeating the Seer creatures in the courtyard using Thinner or Paint when the creatures see you. Head to the ramp on the left, then look to the left to find a red door built into the wall.
Spray the door with Thinner to dissolve it. Open the red chest behind the door to receive an item, the "Dark Beauty Castle Pin."