Mexico 86

On the title screen press and hold the start button, wait 8 seconds exactly and release, do this 3 times and you'll hear a bip-like noise, insert the coin and go to the team selection screen, put the Cursor over Argentina and press and hold start, now choose Argentina, but instead of choosing argentina you'll get the AS Team

While playing a match, press the start button for approximately 4 seconds then release, do this again and the screen will ''flash''. The ball will now be black.