In Kingdom Hearts II, the Monochrome can be obtained in the Timeless River by sealing the door that Pete used to enter the world from Villain's Vale, the place in Radiant Garden where he and Maleficent reside.
The Monochrome's guard is a lifesaver float while the blade is a steamboat stack. The Keychain is a ship's wheel, possibly that of the Steamboat Willie. The teeth also resemble the steam whistles from Steamboat Willie. Other than that, the Monochrome resembles the transmitter of the Graham Bell phone.
The word 'monochrome' is another word for one color, and is often used to describe things that are in black and white, like the world of Timeless River. It could also reference the simple coloration of the Keyblade.