Change background on Super Game Boy
  Enter one of the following codes during game play to change the background to
  match the corresponding Game Boy case colors. You MUST be using a Super Game
  Boy:

  Hold START               Green
  Hold RIGHT + START       Red
  Hold UP + START          Black
  Hold DOWN + START        White
  Hold LEFT + START        Yellow


Change to the transparent Game Boy
  To change to the transparent Game Boy, sweep the D-pad continually in a Full-
  Circle until a tone is heard. Hold the D-pad in that direction and press
  START.

Cheats by Felipe Gerhard - felipe.gerhard@gmx.net