When the game starts loading and it says GAME BOY, you have to press a combination of buttons. There are 12 different combos.
^
>
<
\/
A+^
A+>
A+<
A+\/
B+^
B+>
B+<
B+\/
I cant remember the colours for these except for B+> being inverse, which inverts the colours, so everything that is white is black, yellow turns purple, orange turns blue, red turns, green, pink turns dark green, black turns white, etc.
These all give you different colour coding for the games. HOWEVER, this only works on the Gameboy Advance (SP) when playing Gameboy Pocket games (GRBY were gameboy pocket games, but GSC werent).