Here is all of the unused info I've gathered from Mario Paint so far.
Rotation Options

In addition to the four flip options for stamps and the copy feature, four rotation options are fully programmed into the game. However, they can only be accessed by setting a certain flag in RAM.
PAR code 7E099F01 enables these options:
90° Counterclockwise
90° Clockwise
90° Counterclockwise + Y-flipped
90° Clockwise + Y-flipped
There may be a button combo which sets this flag, but it hasn't been uncovered yet.
Hidden Button Codes
If a controller is plugged into the second controller port, two button combos can be entered to access functions likely used by the developers to test certain features.
Holding Start, A and B enters the game without having to click on Mario.
Holding Select, A and B simultaneously loads and displays the saved painting and animation and plays the saved music. It appears to be a Save RAM test.
If save data exists, "WAIT.." will appear on the screen for a few seconds while the SRAM contents are being loaded.
"NO SAVE DATA" will appear instead if no saved data exists.
Unused Icon

Stored near the graphics for the coloring book icons are similar graphics for an icon depicting a teddy bear. This may mean there was a fifth coloring book picture at one point.
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