On the Title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B. Then before the screen changes, push A or B to alter the color of your opponent's player. A will darken the color while B will keep the colors the same. The code will stay in effect until the power is turned off.
Alter Ego in Single Player Normal Mode
At the Title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A.
Alter Ego in Single Player Mania Mode
At the Title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, B.
Alter Ego in Versus Mode
At the Title screen, press Up, Up, Down, Down, Left, Right, Left, Right, A, B.
Alter Ego in Versus Mania Mode
At the Title screen, press Up, Up, Down, Down, Left, Right, Left, Right, A, A.