When you insert a GB Classic game (e.g. Tetris, Pokémon Red/Blue, Super Mario Land) and the Game Boy logo appears, press the following button combinations to access the associated colour scheme. These change the colour the classic game appears in and some can make old-skool games look very nice.
Up+B- Dark Brown
Down- Pale Yellow
Left+A- Dark Blue
Left+B- Grayscale (Black and White, classic style)
Right+A- Dark Green
Right+B- Negative Image (of automatic colourscheme)
These colour combinations will be reset when the game is turned off. Also, entering one will briefly extend the stay of the title screen, allowing you to change again if you're quick.
To make the screen wide when playing a Gameboy Color game press L, To change it back to normal size press R.
When first turning on the console, press Star and Select at the same time just after the first sound (and the Game Boy logo appears) but before the "ting" and the loadup will pause and a classic Mario sound will be played. Press A to unpause and play another classic Mario sound!
At the starting screen with the gameboy logo hold the(start)and (select)button and the logo will disapear