Instructions
Set up a match and go to the Character Select screen.
Select your character ("A" on both the Wii and GameCube controllers). The character should pop up in a window at the bottom of the screen.
Press the "1" or "2" button on the Wii controller or the "X" or "Y" buttons on the GameCube controller to toggle between the textures. The character's colors will change in the window.
Press "Start" to being the match once you have reached your desired texture color.