The Pro Action Replay code 004C0A:4E71 enables a character editor.
To get to it, hold buttons B and C, and press A. You'll see a bunch of crap appear on the screen and the words "CHARACTER EDITOR 3.52" under everything.
The character editor allows you to modify the VRAM. You can redraw individual sprites and change the color palettes. Press start to restart the game and ignore the changes, or hold B and press start to save the changes and proceed.
Additionally, 004C0A:4E71 allows accessing the editor not only from the SEGA logo, but from anywhere in the game. The interesting thing is, it still resets the game when pressing start rather than going to the memory and level editors.