If you haven't beaten the game, the color of the title screen is normally blue. But after you beat the game, the color switches to purple. You can change it back to blue by pressing Up, Up, Down, Down, Left, Right, Left, Right, B, A at the title screen. Input the same code to change it back to purple.

To do this, you need to beat the game first. Then at any time, press Up, Up, Down, Down, Left, Right, Left, Right, B, A and you'll get to see the credits again.